diff --git a/lib/hooks/thread-hooks.js b/lib/hooks/thread-hooks.js
--- a/lib/hooks/thread-hooks.js
+++ b/lib/hooks/thread-hooks.js
@@ -67,12 +67,15 @@
     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 recipientIDs;
       if (input.type === threadTypes.THICK_SIDEBAR) {
         const { parentThreadID, sourceMessageID, initialMemberIDs } = input;
         invariant(
@@ -92,6 +95,9 @@
           type: 'create_sidebar',
         };
         op = sidebarOP;
+        recipientIDs = threadInfos[parentThreadID].members.map(
+          member => member.id,
+        );
       } else {
         const { type, initialMemberIDs } = input;
 
@@ -106,20 +112,20 @@
           type: 'create_thread',
         };
         op = threadOP;
+        recipientIDs = [...(input.initialMemberIDs ?? []), viewerID];
       }
-      const userIDs = [...(input.initialMemberIDs ?? []), viewerID];
       const opSpecification: OutboundDMOperationSpecification = {
         type: dmOperationSpecificationTypes.OUTBOUND,
         op,
         recipients: {
           type: 'some_users',
-          userIDs,
+          userIDs: recipientIDs,
         },
       };
       await processAndSendDMOperation(opSpecification);
       return threadID;
     },
-    [processAndSendDMOperation, viewerID],
+    [processAndSendDMOperation, threadInfos, viewerID],
   );
 }