diff --git a/lib/shared/dm-ops/add-members-spec.js b/lib/shared/dm-ops/add-members-spec.js
new file mode 100644
--- /dev/null
+++ b/lib/shared/dm-ops/add-members-spec.js
@@ -0,0 +1,82 @@
+// @flow
+
+import uuid from 'uuid';
+
+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 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 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: 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: [addMembersMessage],
+      updateInfos,
+    };
+  },
+});
+
+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';
 
@@ -18,6 +27,7 @@
   SEND_TEXT_MESSAGE: 'send_text_message',
   SEND_REACTION_MESSAGE: 'send_reaction_message',
   SEND_EDIT_MESSAGE: 'send_edit_message',
+  ADD_MEMBERS: 'add_members',
 });
 export type DMOperationType = $Values<typeof dmOperationTypes>;
 
@@ -129,12 +139,37 @@
     text: t.String,
   });
 
+export type DMAddMembersOperation = {
+  +type: 'add_members',
+  +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),
+    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 =
   | DMCreateThreadOperation
   | DMCreateSidebarOperation
   | DMSendTextMessageOperation
   | DMSendReactionMessageOperation
-  | DMSendEditMessageOperation;
+  | DMSendEditMessageOperation
+  | DMAddMembersOperation;
 
 export type DMOperationResult = {
   rawMessageInfos: Array<RawMessageInfo>,