diff --git a/lib/shared/messages/text-message-spec.js b/lib/shared/messages/text-message-spec.js
--- a/lib/shared/messages/text-message-spec.js
+++ b/lib/shared/messages/text-message-spec.js
@@ -9,6 +9,10 @@
   type RawMessageInfoFromServerDBRowParams,
 } from './message-spec.js';
 import { assertSingleMessageInfo } from './utils.js';
+import {
+  changeThreadSettingsActionTypes,
+  changeThreadSettings,
+} from '../../actions/thread-actions.js';
 import { messageTypes } from '../../types/message-types.js';
 import type {
   MessageInfo,
@@ -20,8 +24,13 @@
   TextMessageInfo,
 } from '../../types/messages/text.js';
 import type { NotifTexts } from '../../types/notif-types.js';
+import { threadTypes } from '../../types/thread-types.js';
 import type { ThreadInfo } from '../../types/thread-types.js';
 import type { RelativeUserInfo } from '../../types/user-types.js';
+import {
+  useDispatchActionPromise,
+  useServerCall,
+} from '../../utils/action-utils.js';
 import { ET } from '../../utils/entity-text.js';
 import {
   type ASTNode,
@@ -29,7 +38,7 @@
   stripSpoilersFromNotifications,
   stripSpoilersFromMarkdownAST,
 } from '../markdown.js';
-import { threadIsGroupChat } from '../thread-utils.js';
+import { threadIsGroupChat, extractMentionedMembers } from '../thread-utils.js';
 
 /**
  * most of the markdown leaves contain `content` field
@@ -198,4 +207,46 @@
   generatesNotifs: async () => pushTypes.NOTIF,
 
   includedInRepliesCount: true,
+
+  useCreationSideEffectsFunc: () => {
+    const dispatchActionPromise = useDispatchActionPromise();
+    const callChangeThreadSettings = useServerCall(changeThreadSettings);
+    return async (
+      messageInfo: RawTextMessageInfo,
+      threadInfo: ThreadInfo,
+      parentThreadInfo: ?ThreadInfo,
+    ) => {
+      if (threadInfo.type !== threadTypes.SIDEBAR) {
+        return;
+      }
+      invariant(parentThreadInfo, 'all sidebars should have a parent thread');
+
+      const mentionedMembersOfParent = extractMentionedMembers(
+        messageInfo.text,
+        parentThreadInfo,
+      );
+      if (mentionedMembersOfParent.size === 0) {
+        return;
+      }
+
+      for (const member of threadInfo.members) {
+        if (member.role) {
+          mentionedMembersOfParent.delete(member.id);
+        }
+      }
+
+      const mentionedToAdd = [...mentionedMembersOfParent.keys()];
+      if (mentionedToAdd.length === 0) {
+        return;
+      }
+
+      const addMembersPromise = callChangeThreadSettings({
+        threadID: threadInfo.id,
+        changes: { newMemberIDs: mentionedToAdd },
+      });
+
+      dispatchActionPromise(changeThreadSettingsActionTypes, addMembersPromise);
+      await addMembersPromise;
+    };
+  },
 });
diff --git a/native/input/input-state-container.react.js b/native/input/input-state-container.react.js
--- a/native/input/input-state-container.react.js
+++ b/native/input/input-state-container.react.js
@@ -35,7 +35,9 @@
 import {
   createMediaMessageInfo,
   localIDPrefix,
+  useMessageCreationSideEffectsFunc,
 } from 'lib/shared/message-utils.js';
+import type { CreationSideEffectsFunc } from 'lib/shared/messages/message-spec.js';
 import {
   createRealThreadFromPendingThread,
   threadIsPending,
@@ -138,6 +140,7 @@
     text: string,
   ) => Promise<SendMessageResult>,
   +newThread: (request: ClientNewThreadRequest) => Promise<NewThreadResult>,
+  +textMessageCreationSideEffectsFunc: CreationSideEffectsFunc<RawTextMessageInfo>,
 };
 type State = {
   +pendingUploads: PendingMultimediaUploads,
@@ -466,12 +469,15 @@
 
   async sendTextMessageAction(
     messageInfo: RawTextMessageInfo,
-    // eslint-disable-next-line no-unused-vars
     threadInfo: ThreadInfo,
-    // eslint-disable-next-line no-unused-vars
     parentThreadInfo: ?ThreadInfo,
   ): Promise<SendMessagePayload> {
     try {
+      await this.props.textMessageCreationSideEffectsFunc(
+        messageInfo,
+        threadInfo,
+        parentThreadInfo,
+      );
       const { localID } = messageInfo;
       invariant(
         localID !== null && localID !== undefined,
@@ -1422,6 +1428,9 @@
     const dispatch = useDispatch();
     const mediaReportsEnabled = useIsReportEnabled('mediaReports');
     const staffCanSee = useStaffCanSee();
+    const textMessageCreationSideEffectsFunc = useMessageCreationSideEffectsFunc(
+      messageTypes.TEXT,
+    );
 
     return (
       <InputStateContainer
@@ -1440,6 +1449,7 @@
         dispatchActionPromise={dispatchActionPromise}
         dispatch={dispatch}
         staffCanSee={staffCanSee}
+        textMessageCreationSideEffectsFunc={textMessageCreationSideEffectsFunc}
       />
     );
   });
diff --git a/web/input/input-state-container.react.js b/web/input/input-state-container.react.js
--- a/web/input/input-state-container.react.js
+++ b/web/input/input-state-container.react.js
@@ -37,7 +37,9 @@
 import {
   createMediaMessageInfo,
   localIDPrefix,
+  useMessageCreationSideEffectsFunc,
 } from 'lib/shared/message-utils.js';
+import type { CreationSideEffectsFunc } from 'lib/shared/messages/message-spec.js';
 import {
   createRealThreadFromPendingThread,
   draftKeyFromThreadID,
@@ -125,6 +127,7 @@
   +sendCallbacks: $ReadOnlyArray<() => mixed>,
   +registerSendCallback: (() => mixed) => void,
   +unregisterSendCallback: (() => mixed) => void,
+  +textMessageCreationSideEffectsFunc: CreationSideEffectsFunc<RawTextMessageInfo>,
 };
 type State = {
   +pendingUploads: {
@@ -1046,12 +1049,15 @@
 
   async sendTextMessageAction(
     messageInfo: RawTextMessageInfo,
-    // eslint-disable-next-line no-unused-vars
     threadInfo: ThreadInfo,
-    // eslint-disable-next-line no-unused-vars
     parentThreadInfo: ?ThreadInfo,
   ): Promise<SendMessagePayload> {
     try {
+      await this.props.textMessageCreationSideEffectsFunc(
+        messageInfo,
+        threadInfo,
+        parentThreadInfo,
+      );
       const { localID } = messageInfo;
       invariant(
         localID !== null && localID !== undefined,
@@ -1363,6 +1369,9 @@
       },
       [],
     );
+    const textMessageCreationSideEffectsFunc = useMessageCreationSideEffectsFunc(
+      messageTypes.TEXT,
+    );
 
     return (
       <InputStateContainer
@@ -1384,6 +1393,7 @@
         sendCallbacks={sendCallbacks}
         registerSendCallback={registerSendCallback}
         unregisterSendCallback={unregisterSendCallback}
+        textMessageCreationSideEffectsFunc={textMessageCreationSideEffectsFunc}
       />
     );
   });