diff --git a/src/button/Button.tsx b/src/button/Button.tsx
index 2de62a80..1b517cfc 100644
--- a/src/button/Button.tsx
+++ b/src/button/Button.tsx
@@ -245,6 +245,23 @@ export function SettingsButton({
   );
 }
 
+export function SimulateJoinButton(props: {
+  className?: string;
+  // TODO: add all props for <Button>
+  [index: string]: unknown;
+}) {
+  const { t } = useTranslation();
+  const tooltip = useCallback(() => "Add a fake participant", [t]);
+
+  return (
+    <TooltipTrigger tooltip={tooltip}>
+      <Button variant="toolbar" {...props}>
+        <AddUserIcon width={20} height={20} />
+      </Button>
+    </TooltipTrigger>
+  );
+}
+
 export function InviteButton({
   className,
   variant = "toolbar",
diff --git a/src/room/InCallView.tsx b/src/room/InCallView.tsx
index b8284975..282bfc46 100644
--- a/src/room/InCallView.tsx
+++ b/src/room/InCallView.tsx
@@ -42,6 +42,7 @@ import {
   ScreenshareButton,
   SettingsButton,
   InviteButton,
+  SimulateJoinButton,
 } from "../button";
 import {
   Header,
@@ -58,6 +59,7 @@ import {
 import {
   useShowInspector,
   useShowConnectionStats,
+  useVideoGridSandboxMode,
 } from "../settings/useSetting";
 import { useModalTriggerState } from "../Modal";
 import { PosthogAnalytics } from "../analytics/PosthogAnalytics";
@@ -83,6 +85,8 @@ import { useFullscreen } from "./useFullscreen";
 import { useLayoutStates } from "../video-grid/Layout";
 import { useSFUConfig } from "../livekit/OpenIDLoader";
 import { E2EELock } from "../E2EELock";
+import { useFakeTiles } from "./useFakeTiles";
+import { FakeItemData, FakeVideoTile } from "../video-grid/FakeVideoTile";
 
 const canScreenshare = "getDisplayMedia" in (navigator.mediaDevices ?? {});
 // There is currently a bug in Safari our our code with cloning and sending MediaStreams
@@ -160,6 +164,9 @@ export function InCallView({
 
   const [showInspector] = useShowInspector();
   const [showConnectionStats] = useShowConnectionStats();
+  const [videoGridSandboxMode] = useVideoGridSandboxMode()
+
+  const { fakeTiles: fakeItems, simulateJoin } = useFakeTiles()
 
   const { hideScreensharing } = useUrlParams();
 
@@ -227,9 +234,11 @@ export function InCallView({
   const reducedControls = boundsValid && bounds.width <= 400;
   const noControls = reducedControls && bounds.height <= 400;
 
-  const items = useParticipantTiles(livekitRoom, participants);
+  const realItems = useParticipantTiles(livekitRoom, participants);
   const { fullscreenItem, toggleFullscreen, exitFullscreen } =
-    useFullscreen(items);
+    useFullscreen(realItems);
+
+  const items = useMemo<TileDescriptor<ItemData | FakeItemData>[]>(() => videoGridSandboxMode ? [...realItems, ...fakeItems] : realItems, [videoGridSandboxMode, realItems, fakeItems])
 
   // The maximised participant: either the participant that the user has
   // manually put in fullscreen, or the focused (active) participant if the
@@ -238,9 +247,9 @@ export function InCallView({
     () =>
       fullscreenItem ??
       (noControls
-        ? items.find((item) => item.isSpeaker) ?? items.at(0) ?? null
+        ? realItems.find((item) => item.isSpeaker) ?? realItems.at(0) ?? null
         : null),
-    [fullscreenItem, noControls, items]
+    [fullscreenItem, noControls, realItems]
   );
 
   const Grid =
@@ -283,16 +292,19 @@ export function InCallView({
         disableAnimations={prefersReducedMotion || isSafari}
         layoutStates={layoutStates}
       >
-        {(props) => (
+        {(props) => props.data.type === "real" ? (
           <VideoTile
             maximised={false}
             fullscreen={false}
             onToggleFullscreen={toggleFullscreen}
-            showSpeakingIndicator={items.length > 2}
+            showSpeakingIndicator={realItems.length > 2}
             showConnectionStats={showConnectionStats}
             {...props}
+            data={props.data}
             ref={props.ref as Ref<HTMLDivElement>}
           />
+        ) : (
+          <FakeVideoTile {...props} data={props.data} />
         )}
       </Grid>
     );
@@ -381,6 +393,8 @@ export function InCallView({
         );
       }
       buttons.push(<SettingsButton key="4" onPress={openSettings} />);
+      if (videoGridSandboxMode)
+        buttons.push(<SimulateJoinButton key="5" onPress={simulateJoin} />)
     }
 
     buttons.push(
@@ -480,7 +494,7 @@ function useParticipantTiles(
         const member = matrixParticipants.get(id);
         allGhosts &&= member === undefined;
 
-        const userMediaTile = {
+        const userMediaTile: TileDescriptor<ItemData> = {
           id,
           focused: false,
           isPresenter: sfuParticipant.isScreenShareEnabled,
@@ -491,6 +505,7 @@ function useParticipantTiles(
           local: sfuParticipant.isLocal,
           largeBaseSize: false,
           data: {
+            type: "real",
             id,
             member,
             sfuParticipant,
diff --git a/src/room/useFakeTiles.ts b/src/room/useFakeTiles.ts
new file mode 100644
index 00000000..259fbc8f
--- /dev/null
+++ b/src/room/useFakeTiles.ts
@@ -0,0 +1,96 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { TileDescriptor } from '../video-grid/VideoGrid'
+import { FakeItemData } from '../video-grid/FakeVideoTile'
+import { useCallback, useState } from 'react'
+import { useLatest } from '../useLatest'
+import { generateRandomName } from '../auth/generateRandomName'
+
+interface FakeTiles {
+  fakeTiles: TileDescriptor<FakeItemData>[]
+  simulateJoin: () => void
+}
+
+export const useFakeTiles = (): FakeTiles => {
+  const [tiles, setTiles] = useState<TileDescriptor<FakeItemData>[]>([])
+  const latestSetTiles = useLatest(setTiles)
+
+  const simulateJoin = useCallback(() => {
+    const name = generateRandomName()
+    const newTile: TileDescriptor<FakeItemData> = {
+      id: name,
+      focused: false,
+      isPresenter: false,
+      isSpeaker: false,
+      hasVideo: false,
+      local: false,
+      largeBaseSize: false,
+      data: {
+        type: "fake",
+        name,
+        screenshare: false,
+        speaking: false,
+        simulateScreenshare: () => latestSetTiles.current(ts => {
+          if (ts.some(t => t.data.name === newTile.id && t.data.screenshare)) {
+            // No-op since they're already screensharing
+            return ts
+          } else {
+            const newScreenshareTile: TileDescriptor<FakeItemData> = {
+              id: `${name}:screenshare`,
+              focused: false,
+              isPresenter: false,
+              isSpeaker: false,
+              hasVideo: true,
+              local: false,
+              largeBaseSize: true,
+              placeNear: newTile.id,
+              data: {
+                type: "fake",
+                name,
+                screenshare: true,
+                speaking: false,
+                remove: () => latestSetTiles.current(ts => ts.filter(t => t.id !== newScreenshareTile.id))
+              }
+            }
+            return [...ts, newScreenshareTile]
+          }
+        }),
+        simulateSpeaking: () => latestSetTiles.current(ts => ts.map(t => {
+          if (t.id === newTile.id) {
+            return {
+              ...t,
+              isSpeaker: !t.isSpeaker,
+              data: {
+                ...t.data,
+                speaking: !t.isSpeaker,
+              }
+            }
+          } else {
+            return t
+          }
+        })),
+        remove: () => latestSetTiles.current(ts => ts.filter(t => t.id !== newTile.id))
+      }
+    }
+    setTiles(ts => [...ts, newTile])
+  }, [setTiles, latestSetTiles])
+
+  return {
+    fakeTiles: tiles,
+    simulateJoin,
+  }
+}
diff --git a/src/room/useFullscreen.ts b/src/room/useFullscreen.ts
index 78ec1c87..27952886 100644
--- a/src/room/useFullscreen.ts
+++ b/src/room/useFullscreen.ts
@@ -16,11 +16,12 @@ limitations under the License.
 */
 
 import { logger } from "matrix-js-sdk/src/logger";
-import { useCallback, useLayoutEffect, useRef } from "react";
+import { useCallback, useLayoutEffect } from "react";
 
 import { TileDescriptor } from "../video-grid/VideoGrid";
 import { useReactiveState } from "../useReactiveState";
 import { useEventTarget } from "../useEvents";
+import { useLatest } from "../useLatest";
 
 const isFullscreen = () =>
   Boolean(document.fullscreenElement) ||
@@ -69,11 +70,8 @@ export function useFullscreen<T>(items: TileDescriptor<T>[]): {
       [items]
     );
 
-  const latestItems = useRef<TileDescriptor<T>[]>(items);
-  latestItems.current = items;
-
-  const latestFullscreenItem = useRef<TileDescriptor<T> | null>(fullscreenItem);
-  latestFullscreenItem.current = fullscreenItem;
+  const latestItems = useLatest(items)
+  const latestFullscreenItem = useLatest(fullscreenItem)
 
   const toggleFullscreen = useCallback(
     (itemId: string) => {
diff --git a/src/settings/SettingsModal.tsx b/src/settings/SettingsModal.tsx
index 19379cb4..3423d30e 100644
--- a/src/settings/SettingsModal.tsx
+++ b/src/settings/SettingsModal.tsx
@@ -34,6 +34,7 @@ import {
   useOptInAnalytics,
   useDeveloperSettingsTab,
   useShowConnectionStats,
+  useVideoGridSandboxMode,
 } from "./useSetting";
 import { FieldRow, InputField } from "../input/Input";
 import { Button } from "../button";
@@ -68,6 +69,7 @@ export const SettingsModal = (props: Props) => {
     useDeveloperSettingsTab();
   const [showConnectionStats, setShowConnectionStats] =
     useShowConnectionStats();
+  const [videoGridSandboxMode, setVideoGridSandboxMode] = useVideoGridSandboxMode()
 
   const downloadDebugLog = useDownloadDebugLog();
 
@@ -249,6 +251,18 @@ export const SettingsModal = (props: Props) => {
             }
           />
         </FieldRow>
+        <FieldRow>
+          <InputField
+            id="videoGridSandboxMode"
+            name="video-grid-sandbox-mode"
+            label="Video grid sandbox mode"
+            type="checkbox"
+            checked={videoGridSandboxMode}
+            onChange={(e: ChangeEvent<HTMLInputElement>) =>
+              setVideoGridSandboxMode(e.target.checked)
+            }
+          />
+        </FieldRow>
         <FieldRow>
           <Button onPress={downloadDebugLog}>{t("Download debug logs")}</Button>
         </FieldRow>
diff --git a/src/settings/useSetting.ts b/src/settings/useSetting.ts
index 63f1e23a..61933213 100644
--- a/src/settings/useSetting.ts
+++ b/src/settings/useSetting.ts
@@ -104,6 +104,8 @@ export const useDeveloperSettingsTab = () =>
 export const useShowConnectionStats = () =>
   useSetting("show-connection-stats", false);
 
+export const useVideoGridSandboxMode = () => useSetting("video-grid-sandbox-mode", false)
+
 export const useDefaultDevices = () =>
   useSetting("defaultDevices", {
     audioinput: "",
diff --git a/src/useLatest.ts b/src/useLatest.ts
new file mode 100644
index 00000000..fa54c1e4
--- /dev/null
+++ b/src/useLatest.ts
@@ -0,0 +1,27 @@
+/*
+Copyright 2023 New Vector Ltd
+
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { useRef } from "react"
+
+/**
+ * Returns a ref that always holds the latest value passed to this hook.
+ */
+export const useLatest = <T,>(t: T): { current: T } => {
+  const latest = useRef(t)
+  latest.current = t
+  return latest
+}
diff --git a/src/video-grid/FakeVideoTile.module.css b/src/video-grid/FakeVideoTile.module.css
new file mode 100644
index 00000000..f0444f06
--- /dev/null
+++ b/src/video-grid/FakeVideoTile.module.css
@@ -0,0 +1,42 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.tile {
+  background-color: var(--system);
+  --tileRadius: 20px;
+  border-radius: var(--tileRadius);
+  box-sizing: border-box;
+  padding: 12px;
+  user-select: none;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 12px;
+}
+
+.speaking {
+  border: 4px solid var(--accent);
+}
+
+.name {
+  font-size: var(--font-size-caption);
+}
+
+.buttons {
+  display: flex;
+  gap: 8px;
+}
diff --git a/src/video-grid/FakeVideoTile.tsx b/src/video-grid/FakeVideoTile.tsx
new file mode 100644
index 00000000..8d331481
--- /dev/null
+++ b/src/video-grid/FakeVideoTile.tsx
@@ -0,0 +1,55 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { animated } from "@react-spring/web";
+import classNames from "classnames";
+import { ComponentProps, forwardRef, Ref } from "react"
+import { Avatar } from "../Avatar";
+import { ReactComponent as HangupIcon } from "../icons/Hangup.svg";
+import { ReactComponent as ScreenshareIcon } from "../icons/Screenshare.svg";
+import { ReactComponent as MicIcon } from "../icons/Mic.svg";
+import styles from "./FakeVideoTile.module.css";
+
+
+export interface FakeItemData {
+  type: "fake", // To differentiate from ItemData
+  name: string,
+  screenshare: boolean,
+  speaking: boolean,
+  simulateScreenshare?: () => void
+  simulateSpeaking?: () => void
+  remove: () => void
+}
+
+interface Props {
+  data: FakeItemData
+  className?: string;
+  style?: ComponentProps<typeof animated.div>["style"];
+}
+
+export const FakeVideoTile = forwardRef<HTMLElement, Props>(({ data, className, style }, ref) => {
+  return <animated.div className={classNames(className, styles.tile, { [styles.speaking]: data.speaking })} ref={ref as Ref<HTMLDivElement>} style={style}>
+    <Avatar fallback={data.name[0].toUpperCase()} />
+    <div className={styles.name}>
+      {data.screenshare ? `${data.name} (sharing screen)` : data.name}
+    </div>
+    <div className={styles.buttons}>
+      {data.simulateSpeaking && <MicIcon onClick={data.simulateSpeaking} />}
+      {data.simulateScreenshare && <ScreenshareIcon onClick={data.simulateScreenshare} />}
+      <HangupIcon onClick={data.remove} />
+    </div>
+  </animated.div>
+})
diff --git a/src/video-grid/TileWrapper.module.css b/src/video-grid/TileWrapper.module.css
new file mode 100644
index 00000000..17d26c25
--- /dev/null
+++ b/src/video-grid/TileWrapper.module.css
@@ -0,0 +1,23 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.tile {
+  position: absolute;
+  contain: strict;
+  overflow: hidden;
+  cursor: pointer;
+  top: 0;
+}
diff --git a/src/video-grid/TileWrapper.tsx b/src/video-grid/TileWrapper.tsx
index d1712ce5..bf5ed7c3 100644
--- a/src/video-grid/TileWrapper.tsx
+++ b/src/video-grid/TileWrapper.tsx
@@ -19,6 +19,7 @@ import { EventTypes, Handler, useDrag } from "@use-gesture/react";
 import { SpringValue, to } from "@react-spring/web";
 
 import { ChildrenProperties } from "./VideoGrid";
+import styles from "./TileWrapper.module.css"
 
 interface Props<T> {
   id: string;
@@ -77,6 +78,7 @@ export const TileWrapper = memo(
       <>
         {children({
           ref,
+          className: styles.tile,
           style: {
             opacity,
             scale,
diff --git a/src/video-grid/VideoGrid.tsx b/src/video-grid/VideoGrid.tsx
index 77393b8b..a96687cf 100644
--- a/src/video-grid/VideoGrid.tsx
+++ b/src/video-grid/VideoGrid.tsx
@@ -802,6 +802,7 @@ interface DragTileData {
 
 export interface ChildrenProperties<T> {
   ref: Ref<HTMLElement>;
+  className?: string;
   style: ComponentProps<typeof animated.div>["style"];
   /**
    * The width this tile will have once its animations have settled.
diff --git a/src/video-grid/VideoTile.module.css b/src/video-grid/VideoTile.module.css
index 20849a94..f579e5c5 100644
--- a/src/video-grid/VideoTile.module.css
+++ b/src/video-grid/VideoTile.module.css
@@ -15,15 +15,11 @@ limitations under the License.
 */
 
 .videoTile {
-  position: absolute;
-  contain: strict;
-  top: 0;
   container-name: videoTile;
   container-type: size;
   --tileRadius: 8px;
   border-radius: var(--tileRadius);
   overflow: hidden;
-  cursor: pointer;
 
   /* HACK: This has no visual effect due to the short duration, but allows the
   JS to detect movement via the transform property for audio spatialization */
@@ -154,7 +150,7 @@ limitations under the License.
 .videoMutedOverlay {
   width: 100%;
   height: 100%;
-  background-color: #21262c;
+  background-color: var(--system);
 }
 
 .presenterLabel {
diff --git a/src/video-grid/VideoTile.tsx b/src/video-grid/VideoTile.tsx
index 63ceddf2..30486743 100644
--- a/src/video-grid/VideoTile.tsx
+++ b/src/video-grid/VideoTile.tsx
@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { ComponentProps, forwardRef, useCallback, useEffect } from "react";
+import { ComponentProps, forwardRef, Ref, useCallback, useEffect } from "react";
 import { animated } from "@react-spring/web";
 import classNames from "classnames";
 import { useTranslation } from "react-i18next";
@@ -39,6 +39,7 @@ import { useModalTriggerState } from "../Modal";
 import { VideoTileSettingsModal } from "./VideoTileSettingsModal";
 
 export interface ItemData {
+  type: "real", // To differentiate from FakeItemData
   id: string;
   member?: RoomMember;
   sfuParticipant: LocalParticipant | RemoteParticipant;
@@ -64,7 +65,7 @@ interface Props {
   showConnectionStats: boolean;
 }
 
-export const VideoTile = forwardRef<HTMLDivElement, Props>(
+export const VideoTile = forwardRef<HTMLElement, Props>(
   (
     {
       data,
@@ -159,7 +160,7 @@ export const VideoTile = forwardRef<HTMLDivElement, Props>(
           [styles.maximised]: maximised,
         })}
         style={style}
-        ref={tileRef}
+        ref={tileRef as Ref<HTMLDivElement>}
         data-testid="videoTile"
       >
         {toolbarButtons.length > 0 && (!maximised || fullscreen) && (