Extra permission - assign user roles

It would be great if there was an extra permission that allowed the holder of the permission to assign a user role to existing users, but did not give them full access to the access controls.

This way a site manager (not admin) could add users and assign them roles without being able to modify the actual permissions for the role.

I would propose an extra permission level "assign user roles" that would give them access to user roles in the user admin page, and is seperate from 'administer access control".

Or perhaps this is already possible?