Posting in a Client project

After you've added a Client to the project, every item is private by default. A Client added to a project won't see anything in it unless you change the visibility of that item.

When you post something, you'll be asked to decide whether it's private to the team or visible to the Client:

While "Our team only — private to us" is selected, you won't be able to @mention or add Client users as subscribers. If you select "Allow the client to see this too," you can @mention Client users and Clients will be added to the list of potential subscribers: