diff --git a/lib/hooks/thread-search-hooks.js b/lib/hooks/thread-search-hooks.js
--- a/lib/hooks/thread-search-hooks.js
+++ b/lib/hooks/thread-search-hooks.js
@@ -103,12 +103,15 @@
     state => state.currentUserInfo && state.currentUserInfo.id,
   );
 
+  const threadInfos = useSelector(state => state.threadStore.threadInfos);
+
   return React.useCallback(
     async (input: NewThickThreadRequest) => {
       invariant(viewerID, 'viewerID must be defined');
       const threadID = input.id ?? uuid.v4();
 
       let op;
+      let userIDs;
       if (input.type === threadTypes.THICK_SIDEBAR) {
         const { parentThreadID, sourceMessageID, initialMemberIDs } = input;
         invariant(
@@ -128,6 +131,11 @@
           type: 'create_sidebar',
         };
         op = sidebarOP;
+        const memberIDs = [...(input.initialMemberIDs ?? []), viewerID];
+        userIDs = [
+          ...memberIDs,
+          ...threadInfos[parentThreadID].members.map(member => member.id),
+        ];
       } else {
         const { type, initialMemberIDs } = input;
 
@@ -142,8 +150,8 @@
           type: 'create_thread',
         };
         op = threadOP;
+        userIDs = [...(input.initialMemberIDs ?? []), viewerID];
       }
-      const userIDs = [...(input.initialMemberIDs ?? []), viewerID];
       const opSpecification: OutboundDMOperationSpecification = {
         type: dmOperationSpecificationTypes.OUTBOUND,
         op,
@@ -155,7 +163,7 @@
       await processAndSendDMOperation(opSpecification);
       return threadID;
     },
-    [processAndSendDMOperation, viewerID],
+    [processAndSendDMOperation, threadInfos, viewerID],
   );
 }