diff --git a/lib/shared/dm-ops/join-thread-spec.js b/lib/shared/dm-ops/join-thread-spec.js
--- a/lib/shared/dm-ops/join-thread-spec.js
+++ b/lib/shared/dm-ops/join-thread-spec.js
@@ -50,22 +50,56 @@
 
     const currentThreadInfo =
       utilities.threadInfos[existingThreadDetails.threadID];
+    if (currentThreadInfo && !currentThreadInfo.thick) {
+      return {
+        rawMessageInfos: [],
+        updateInfos: [],
+      };
+    }
 
     const messageData = createMessageDataFromDMOperation(dmOperation);
     const joinThreadMessageInfos = [
       rawMessageInfoFromMessageData(messageData, messageID),
     ];
 
-    if (userIsMember(currentThreadInfo, joinerID)) {
+    const memberTimestamps = { ...currentThreadInfo?.timestamps?.members };
+    if (!memberTimestamps[joinerID]) {
+      memberTimestamps[joinerID] = {
+        isMember: time,
+        subscription: existingThreadDetails.creationTime,
+      };
+    }
+
+    if (memberTimestamps[joinerID].isMember > time) {
       return {
         rawMessageInfos: joinThreadMessageInfos,
         updateInfos: [],
       };
     }
 
+    memberTimestamps[joinerID] = {
+      ...memberTimestamps[joinerID],
+      isMember: time,
+    };
+
     const updateInfos: Array<ClientUpdateInfo> = [];
     const rawMessageInfos: Array<RawMessageInfo> = [];
-    if (viewerID === joinerID) {
+
+    if (userIsMember(currentThreadInfo, joinerID)) {
+      rawMessageInfos.push(...joinThreadMessageInfos);
+      updateInfos.push({
+        type: updateTypes.UPDATE_THREAD,
+        id: uuid.v4(),
+        time,
+        threadInfo: {
+          ...currentThreadInfo,
+          timestamps: {
+            ...currentThreadInfo.timestamps,
+            members: memberTimestamps,
+          },
+        },
+      });
+    } else if (viewerID === joinerID) {
       const newThreadInfo = createThickRawThreadInfo(
         {
           ...existingThreadDetails,
@@ -80,14 +114,18 @@
         type: updateTypes.JOIN_THREAD,
         id: uuid.v4(),
         time,
-        threadInfo: newThreadInfo,
+        threadInfo: {
+          ...newThreadInfo,
+          timestamps: {
+            ...newThreadInfo.timestamps,
+            members: memberTimestamps,
+          },
+        },
         rawMessageInfos: joinThreadMessageInfos,
         truncationStatus: messageTruncationStatus.EXHAUSTIVE,
         rawEntryInfos: [],
       });
     } else {
-      invariant(currentThreadInfo.thick, 'Thread should be thick');
-
       rawMessageInfos.push(...joinThreadMessageInfos);
       const defaultRoleID = values(currentThreadInfo.roles).find(role =>
         roleIsDefaultRole(role),
@@ -109,6 +147,10 @@
       const updatedThreadInfo = {
         ...currentThreadInfo,
         members: [...currentThreadInfo.members, member],
+        timestamps: {
+          ...currentThreadInfo.timestamps,
+          members: memberTimestamps,
+        },
       };
       updateInfos.push({
         type: updateTypes.UPDATE_THREAD,
@@ -127,7 +169,10 @@
     viewerID: string,
     utilities: ProcessDMOperationUtilities,
   ) {
-    if (utilities.threadInfos[dmOperation.existingThreadDetails.threadID]) {
+    if (
+      utilities.threadInfos[dmOperation.existingThreadDetails.threadID] ||
+      dmOperation.joinerID === viewerID
+    ) {
       return { isProcessingPossible: true };
     }
     return {