User Management

Hi,
i am looking for a plugin or method that will help with the following

I have a 5 site network (may be as many as 7)
Users register through a common registration form on the base site
the site admin needs to add specific users to specific sites (a user may belong to only one site)

I am looking for a plugin that will add a quick method for adding a usr to one of the sites, something like a dropdown on the user profile page or user listing page

Thanks