How to change the role from viewer to user?

Somehow one of our guys is a viewer on the project instead of a user and so cannot be assigned tasks. The HELP feature says either he changes his role, or as admin I can do it. If I am in the project and go to followers, I can see him there, but editing him can only change his role from manager to something else. I told him that he would have to do it, but we cannot find any place to do so.

Thank you