diff --git a/packages/workshop-backend/src/external-message-gateway.ts b/packages/workshop-backend/src/external-message-gateway.ts
index 6a48a5a9b..1878ced15 100644
--- a/packages/workshop-backend/src/external-message-gateway.ts
+++ b/packages/workshop-backend/src/external-message-gateway.ts
@@ -1,6 +1,8 @@
 import { WorkerEntrypoint } from "cloudflare:workers";
 import { validateRpc } from "capnweb-validate";
+import type { CollaboratorInfo } from "@gadgets/workshop-shared/api";
 import {
+  type AddExternalCollaboratorInput,
   type ExternalMessageGateway as ExternalMessageGatewayContract,
   type SubmitExternalMessageInput,
   type SubmitExternalMessageResult,
@@ -36,4 +38,18 @@ export class ExternalMessageGateway extends WorkerEntrypoint<Cloudflare.Env, Ext
       title: input.gadgetTitle,
     });
   }
+
+  async addCollaborator(input: AddExternalCollaboratorInput): Promise<CollaboratorInfo | null> {
+    let source = this.ctx.props.source;
+    if (!source) throw new Error("ExternalMessageGateway source prop is required.");
+
+    // Same DO name as submitExternalMessage — gadgetKey is gateway-namespaced.
+    let overseer = this.ctx.exports.OverseerDurableObject.getByName(`${source}:${input.gadgetKey}`);
+
+    return await overseer.addExternalCollaborator({
+      username: input.username,
+      role: input.role,
+      note: input.note,
+    });
+  }
 }
diff --git a/packages/workshop-backend/src/overseer.ts b/packages/workshop-backend/src/overseer.ts
index d6267e31f..ef1e5ef92 100644
--- a/packages/workshop-backend/src/overseer.ts
+++ b/packages/workshop-backend/src/overseer.ts
@@ -6612,6 +6612,43 @@ export class OverseerDurableObject extends DurableObject<Cloudflare.Env> {
     return { accepted: true, chatPath: `/workspace/${this.ctx.id.toString()}?chat=${chatId}` };
   }
 
+  /**
+   * Grant Build (or another role) to an existing account on behalf of the workspace owner.
+   *
+   * Used by ExternalMessageGateway when a Teams (or other) gateway has already authenticated the
+   * username. This is not the session-bound Overseer.addCollaborator path — there is no browser
+   * client here — so the share is attributed to the owner. Returns null when the username has no
+   * account (same contract as Overseer.addCollaborator). Throws when sharing is prohibited.
+   */
+  async addExternalCollaborator(input: {
+    username: string;
+    role: CollaboratorRole;
+    note?: string;
+  }): Promise<CollaboratorInfo | null> {
+    let ownerProfileId = this.impl.ownerProfileId;
+    if (!ownerProfileId) {
+      // Workspace has not been claimed yet; there is nothing to share into.
+      return null;
+    }
+
+    let userDo = this.impl.users.get(this.impl.users.idFromName(input.username));
+    let profile = await userDo.whoamiIfExists();
+    if (!profile) return null;
+
+    if (this.impl.storage.prohibitAllSharing.get()) {
+      throw new Error(
+          "This workspace has observed sensitive data. To prevent leaks, the workspace cannot be " +
+          "shared.");
+    }
+
+    return (await this.impl.getSharingManager()).addCollaborator({
+      caller: { profileId: ownerProfileId, isOwner: true },
+      profile,
+      role: input.role,
+      note: input.note,
+    });
+  }
+
   // Initialize this workspace's default gadget from a blueprint's code snapshot. Called by
   // AuthenticatedApi.newGadgetFromBlueprint() after creating (and opening) the DO.
   async initializeFromBlueprint(code: Uint8Array, title: string, output?: BlueprintOutput)
diff --git a/packages/workshop-shared/src/external-message-gateway.ts b/packages/workshop-shared/src/external-message-gateway.ts
index 875e63a41..d1b280c5d 100644
--- a/packages/workshop-shared/src/external-message-gateway.ts
+++ b/packages/workshop-shared/src/external-message-gateway.ts
@@ -1,4 +1,5 @@
 import type { RpcStub, RpcTarget } from "cloudflare:workers";
+import type { CollaboratorInfo, CollaboratorRole } from "./api.ts";
 
 /** A completed Gadget response that should be delivered back to the chat gateway. */
 export type GadgetResponse = {
@@ -45,8 +46,27 @@ export type SubmitExternalMessageResult =
       message: string;
     };
 
+/** Grant a role to an existing account on a gateway-owned workspace. */
+export type AddExternalCollaboratorInput = {
+  // Selects the workspace (same gadgetKey namespace as submitExternalMessage).
+  gadgetKey: string;
+  // Username/email the gateway has already authenticated. Never accept a model- or
+  // client-supplied identity here — the gateway is the trusted boundary.
+  username: string;
+  role: CollaboratorRole;
+  note?: string;
+};
+
 /** Service binding RPC interface used by chat gateway workers. */
 export interface ExternalMessageGateway {
   /** Submit an external chat message for Gadget routing and execution. */
   submitExternalMessage(input: SubmitExternalMessageInput): Promise<SubmitExternalMessageResult>;
+
+  /**
+   * Add a collaborator to a gateway-owned workspace, acting as the workspace owner.
+   *
+   * Returns null when `username` has no account. Gateways must only pass usernames they have
+   * authenticated (e.g. Teams roster → Graph email). Throws when sharing is prohibited.
+   */
+  addCollaborator(input: AddExternalCollaboratorInput): Promise<CollaboratorInfo | null>;
 }