diff --git a/lib/shared/thread-utils.js b/lib/shared/thread-utils.js
--- a/lib/shared/thread-utils.js
+++ b/lib/shared/thread-utils.js
@@ -441,6 +441,39 @@
   };
 }
 
+// Returns map from lowercase username to AccountUserInfo
+function memberLowercaseUsernameMap(
+  members: $ReadOnlyArray<RelativeMemberInfo>,
+): Map<string, AccountUserInfo> {
+  const memberMap = new Map();
+  for (const member of members) {
+    const { id, role, username } = member;
+    if (!role || !username) {
+      continue;
+    }
+    memberMap.set(username.toLowerCase(), { id, username });
+  }
+  return memberMap;
+}
+
+// Returns map from user ID to AccountUserInfo
+function extractMentionedMembers(
+  text: string,
+  threadInfo: ThreadInfo,
+): Map<string, AccountUserInfo> {
+  const memberMap = memberLowercaseUsernameMap(threadInfo.members);
+  const mentions = extractMentionsFromText(text);
+
+  const mentionedMembers = new Map();
+  for (const mention of mentions) {
+    const userInfo = memberMap.get(mention.toLowerCase());
+    if (userInfo) {
+      mentionedMembers.set(userInfo.id, userInfo);
+    }
+  }
+  return mentionedMembers;
+}
+
 type SharedCreatePendingSidebarInput = {
   +sourceMessageInfo: ComposableMessageInfo | RobotextMessageInfo,
   +parentThreadInfo: ThreadInfo,
@@ -499,20 +532,12 @@
   }
 
   if (sourceMessageInfo.type === messageTypes.TEXT) {
-    const memberMap = new Map();
-    for (const member of parentThreadInfo.members) {
-      const { id, role, username } = member;
-      if (!role || !username) {
-        continue;
-      }
-      memberMap.set(username.toLowerCase(), { id, username });
-    }
-    const mentions = extractMentionsFromText(sourceMessageInfo.text);
-    for (const mention of mentions) {
-      const userInfo = memberMap.get(mention.toLowerCase());
-      if (userInfo) {
-        initialMembers.set(userInfo.id, userInfo);
-      }
+    const mentionedMembersOfParent = extractMentionedMembers(
+      sourceMessageInfo.text,
+      parentThreadInfo,
+    );
+    for (const [memberID, member] of mentionedMembersOfParent) {
+      initialMembers.set(memberID, member);
     }
   }
 
@@ -1524,6 +1549,7 @@
   parsePendingThreadID,
   createPendingThread,
   createUnresolvedPendingSidebar,
+  extractMentionedMembers,
   createPendingSidebar,
   pendingThreadType,
   createRealThreadFromPendingThread,