Another not entirely satisfactory reason for no "add user" button: adding a user to the server doesn't automatically add them to any groups. "Project users" is defined as any user who is a member of any group in that project. If "add user" appeared on the project users list, a project admin would add the user but they wouldn't appear in the project users list... which would be pretty confusing. As Matt said, we could stand to review the concept of "project users"...

Adam