diff --git a/lib/handlers/initial-state-sharing-handler.react.js b/lib/handlers/initial-state-sharing-handler.react.js
new file mode 100644
--- /dev/null
+++ b/lib/handlers/initial-state-sharing-handler.react.js
@@ -0,0 +1,89 @@
+// @flow
+
+import invariant from 'invariant';
+import * as React from 'react';
+
+import { getOwnPeerDevices } from '../selectors/user-selectors.js';
+import {
+  dmOperationSpecificationTypes,
+  getCreateThickRawThreadInfoInputFromThreadInfo,
+} from '../shared/dm-ops/dm-op-utils.js';
+import { useProcessAndSendDMOperation } from '../shared/dm-ops/process-dm-ops.js';
+import { IdentityClientContext } from '../shared/identity-client-context.js';
+import { values } from '../utils/objects.js';
+import { useSelector } from '../utils/redux-utils.js';
+
+function InitialStateSharingHandler(): React.Node {
+  const userDeviceIDs = useSelector(state =>
+    getOwnPeerDevices(state).map(peer => peer.deviceID),
+  );
+  const threadInfos = useSelector(state => state.threadStore.threadInfos);
+  const viewerID = useSelector(
+    state => state.currentUserInfo && state.currentUserInfo.id,
+  );
+  const identityContext = React.useContext(IdentityClientContext);
+  const processAndSendDMOperation = useProcessAndSendDMOperation();
+
+  const deviceIDs = React.useRef<$ReadOnlySet<string>>(new Set(userDeviceIDs));
+
+  React.useEffect(() => {
+    void (async () => {
+      const oldDeviceIDs = deviceIDs.current;
+      const newDeviceIDs = new Set(userDeviceIDs);
+      deviceIDs.current = newDeviceIDs;
+
+      invariant(identityContext, 'Identity context should be set');
+      const authMetadata = await identityContext.getAuthMetadata();
+      if (authMetadata.deviceID !== userDeviceIDs[0] || !viewerID) {
+        return;
+      }
+
+      const recipients = new Set<string>();
+      for (const deviceID of newDeviceIDs) {
+        if (!oldDeviceIDs.has(deviceID)) {
+          recipients.add(deviceID);
+        }
+      }
+
+      if (recipients.size === 0) {
+        return;
+      }
+
+      for (const threadInfo of values(threadInfos)) {
+        if (!threadInfo.thick) {
+          continue;
+        }
+
+        const existingThreadDetails =
+          getCreateThickRawThreadInfoInputFromThreadInfo(threadInfo);
+        const operation = {
+          type: 'add_viewer_to_thread_members',
+          existingThreadDetails,
+          editorID: viewerID,
+          time: threadInfo.creationTime,
+          messageID: null,
+          addedUserIDs: [],
+        };
+        void processAndSendDMOperation({
+          type: dmOperationSpecificationTypes.OUTBOUND,
+          op: operation,
+          recipients: {
+            type: 'some_devices',
+            deviceIDs: [...recipients],
+          },
+          sendOnly: true,
+        });
+      }
+    })();
+  }, [
+    identityContext,
+    processAndSendDMOperation,
+    threadInfos,
+    userDeviceIDs,
+    viewerID,
+  ]);
+
+  return null;
+}
+
+export { InitialStateSharingHandler };
diff --git a/lib/shared/dm-ops/add-viewer-to-thread-members-spec.js b/lib/shared/dm-ops/add-viewer-to-thread-members-spec.js
--- a/lib/shared/dm-ops/add-viewer-to-thread-members-spec.js
+++ b/lib/shared/dm-ops/add-viewer-to-thread-members-spec.js
@@ -124,7 +124,13 @@
       dmOperation: DMAddViewerToThreadMembersOperation,
       viewerID: string,
     ) {
-      if (dmOperation.addedUserIDs.includes(viewerID)) {
+      // We expect the viewer to be in the added users when the DM op
+      // is processed. An exception is for ops generated
+      // by InitialStateSharingHandler, which won't contain a messageID
+      if (
+        dmOperation.addedUserIDs.includes(viewerID) ||
+        !dmOperation.messageID
+      ) {
         return { isProcessingPossible: true };
       }
       console.log('Invalid DM operation', dmOperation);
diff --git a/lib/shared/dm-ops/dm-op-utils.js b/lib/shared/dm-ops/dm-op-utils.js
--- a/lib/shared/dm-ops/dm-op-utils.js
+++ b/lib/shared/dm-ops/dm-op-utils.js
@@ -245,4 +245,8 @@
   );
 }
 
-export { createMessagesToPeersFromDMOp, useAddDMThreadMembers };
+export {
+  createMessagesToPeersFromDMOp,
+  useAddDMThreadMembers,
+  getCreateThickRawThreadInfoInputFromThreadInfo,
+};
diff --git a/native/root.react.js b/native/root.react.js
--- a/native/root.react.js
+++ b/native/root.react.js
@@ -36,6 +36,7 @@
 import { StaffContextProvider } from 'lib/components/staff-provider.react.js';
 import { UserIdentityCacheProvider } from 'lib/components/user-identity-cache.react.js';
 import { DBOpsHandler } from 'lib/handlers/db-ops-handler.react.js';
+import { InitialStateSharingHandler } from 'lib/handlers/initial-state-sharing-handler.react.js';
 import { TunnelbrokerDeviceTokenHandler } from 'lib/handlers/tunnelbroker-device-token-handler.react.js';
 import { UserInfosHandler } from 'lib/handlers/user-infos-handler.react.js';
 import { IdentitySearchProvider } from 'lib/identity-search/identity-search-context.js';
@@ -381,6 +382,7 @@
                                                     <PrekeysHandler />
                                                     <ReportHandler />
                                                     <AutoJoinCommunityHandler />
+                                                    <InitialStateSharingHandler />
                                                   </PersistedStateGate>
                                                   {navigation}
                                                 </RegistrationContextProvider>