diff --git a/lib/shared/dm-ops/add-members-spec.js b/lib/shared/dm-ops/add-members-spec.js
--- a/lib/shared/dm-ops/add-members-spec.js
+++ b/lib/shared/dm-ops/add-members-spec.js
@@ -2,45 +2,81 @@
 
 import uuid from 'uuid';
 
-import type { DMOperationSpec } from './dm-op-spec.js';
-import type { DMSendEditMessageOperation } from '../../types/dm-ops.js';
+import type {
+  DMOperationSpec,
+  ProcessDMOperationUtilities,
+} from './dm-op-spec.js';
+import type { DMAddMembersOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
 import { updateTypes } from '../../types/update-types-enum.js';
 import type { ClientUpdateInfo } from '../../types/update-types.js';
 
-const sendEditMessageSpec: DMOperationSpec<DMSendEditMessageOperation> =
-  Object.freeze({
-    processDMOperation: async (
-      dmOperation: DMSendEditMessageOperation,
-      viewerID: string,
-    ) => {
-      const { threadID, creatorID, time, messageID, targetMessageID, text } =
-        dmOperation;
-      const reactionMessage = {
-        type: messageTypes.EDIT_MESSAGE,
-        id: messageID,
-        threadID,
-        creatorID,
-        time,
-        targetMessageID,
-        text,
-      };
+const addMembersSpec: DMOperationSpec<DMAddMembersOperation> = Object.freeze({
+  processDMOperation: async (
+    dmOperation: DMAddMembersOperation,
+    viewerID: string,
+    utilities: ProcessDMOperationUtilities,
+  ) => {
+    const {
+      editorID,
+      time,
+      messageID,
+      addedUserIDs,
+      threadInfo,
+      rawMessageInfos,
+      truncationStatus,
+      rawEntryInfos,
+    } = dmOperation;
+    const addMembersMessage = {
+      type: messageTypes.ADD_MEMBERS,
+      id: messageID,
+      threadID: threadInfo.id,
+      creatorID: editorID,
+      time,
+      addedUserIDs: [...addedUserIDs],
+    };
 
-      const updateInfos: Array<ClientUpdateInfo> = [];
-      if (creatorID !== viewerID) {
-        updateInfos.push({
+    const viewerIsAdded = addedUserIDs.includes(viewerID);
+    const updateInfos: Array<ClientUpdateInfo> = [];
+    if (viewerIsAdded) {
+      updateInfos.push(
+        {
+          type: updateTypes.JOIN_THREAD,
+          id: uuid.v4(),
+          time,
+          threadInfo,
+          rawMessageInfos,
+          truncationStatus,
+          rawEntryInfos,
+        },
+        {
           type: updateTypes.UPDATE_THREAD_READ_STATUS,
           id: uuid.v4(),
           time,
-          threadID,
+          threadID: threadInfo.id,
           unread: true,
+        },
+      );
+    } else {
+      const currentThreadInfo = await utilities.fetchThread(threadInfo.id);
+      if (currentThreadInfo?.thick) {
+        const newThreadInfo = {
+          ...currentThreadInfo,
+          members: threadInfo.members,
+        };
+        updateInfos.push({
+          type: updateTypes.UPDATE_THREAD,
+          id: uuid.v4(),
+          time,
+          threadInfo: newThreadInfo,
         });
       }
-      return {
-        rawMessageInfos: [reactionMessage],
-        updateInfos,
-      };
-    },
-  });
+    }
+    return {
+      rawMessageInfos: [addMembersMessage],
+      updateInfos,
+    };
+  },
+});
 
-export { sendEditMessageSpec };
+export { addMembersSpec };
diff --git a/lib/shared/dm-ops/dm-op-spec.js b/lib/shared/dm-ops/dm-op-spec.js
--- a/lib/shared/dm-ops/dm-op-spec.js
+++ b/lib/shared/dm-ops/dm-op-spec.js
@@ -2,10 +2,12 @@
 
 import type { DMOperation, DMOperationResult } from '../../types/dm-ops.js';
 import type { RawMessageInfo } from '../../types/message-types.js';
+import type { RawThreadInfo } from '../../types/minimally-encoded-thread-permissions-types.js';
 
 export type ProcessDMOperationUtilities = {
   // Needed to fetch sidebar source messages
   +fetchMessage: (messageID: string) => Promise<?RawMessageInfo>,
+  fetchThread: (threadID: string) => Promise<?RawThreadInfo>,
 };
 
 export type DMOperationSpec<DMOp: DMOperation> = {
diff --git a/lib/types/dm-ops.js b/lib/types/dm-ops.js
--- a/lib/types/dm-ops.js
+++ b/lib/types/dm-ops.js
@@ -1,14 +1,23 @@
 // @flow
 
-import type { TInterface } from 'tcomb';
-import t from 'tcomb';
+import t, { type TInterface } from 'tcomb';
 
-import type { RawMessageInfo } from './message-types.js';
+import { type RawEntryInfo, rawEntryInfoValidator } from './entry-types.js';
+import type {
+  MessageTruncationStatus,
+  RawMessageInfo,
+} from './message-types.js';
+import {
+  messageTruncationStatusValidator,
+  rawMessageInfoValidator,
+} from './message-types.js';
+import type { ThickRawThreadInfo } from './minimally-encoded-thread-permissions-types.js';
 import {
   type NonSidebarThickThreadType,
   nonSidebarThickThreadTypes,
 } from './thread-types-enum.js';
 import type { ClientUpdateInfo } from './update-types.js';
+import { threadInfoValidator } from '../permissions/minimally-encoded-thread-permissions-validators.js';
 import { values } from '../utils/objects.js';
 import { tShape, tString, tUserID } from '../utils/validation-utils.js';
 
@@ -132,20 +141,26 @@
 
 export type DMAddMembersOperation = {
   +type: 'add_members',
-  +threadID: string,
   +editorID: string,
   +time: number,
   +messageID: string,
   +addedUserIDs: $ReadOnlyArray<string>,
+  +threadInfo: ThickRawThreadInfo,
+  +rawMessageInfos: $ReadOnlyArray<RawMessageInfo>,
+  +truncationStatus: MessageTruncationStatus,
+  +rawEntryInfos: $ReadOnlyArray<RawEntryInfo>,
 };
 export const dmAddMembersOperation: TInterface<DMAddMembersOperation> =
   tShape<DMAddMembersOperation>({
     type: tString(dmOperationTypes.ADD_MEMBERS),
-    threadID: t.String,
     editorID: tUserID,
     time: t.Number,
     messageID: t.String,
     addedUserIDs: t.list(tUserID),
+    threadInfo: threadInfoValidator,
+    rawMessageInfos: t.list(rawMessageInfoValidator),
+    truncationStatus: messageTruncationStatusValidator,
+    rawEntryInfos: t.list(rawEntryInfoValidator),
   });
 
 export type DMOperation =