How can I add a user to a Workspace

Invite Users option is to send an email to that particular user. You cannot invite "another" user. WorkSpaces are dedicated persistent desktops. You have one WorkSpace per user per directory. Once a WorkSpaces is created for a user, you cannot add another user to it.

If you are looking for solution which allows you to create a "pool of desktops" and invite users to consume from the pool, you may explore AppStream "Desktop Mode". But AppStream is non-persistent solution and totally different service from WorkSpaces.