WordPress MU

After much searching I've been unable to identify a plugin or enhancement for WPMU that will provide the user management capabilities we need for our site.

CONTEXT

We have three levels of user - admin, facilitator, and student.

The admin (1) - can see and participate in all blogs on the site;
The facilitator (say 2 - 5 of these on the site)
Users - 10 - 20 per blog.

There is only one admin for this site. Each blog created by the admin is private, with membership by emailed invitation only. Users cannot register for a blog themselves.

Each blog is used as an e-learning space by a group of clients. Each group will be made up of users who belong only to one blog. The Facilitator level role

ISSUE

The problem is that we would like facilitators to be able to log into a number of blogs, but using the same username and password combo for all of them. When trying to assign an existing facilitator to a new group, I'm getting errors that the user already exists. Is this possible to resolve this problem?

Also can anyone point me to a good plugin that is MU compatible and would simplify centralized user registrations and email management workflow - given the way we are using WPMU? We have tried the DD-Import Users plugin currently for this but would like something that gives us more flexibility.

Thanks for any assistance.

If they are a user on the system - no matter what their level - they can log into any blog they are added to.

So, if you have a facilitator on the Art History blog, and they need to have duties on the English Lit pre-1850 blog ;) , the admin of the English blog needs to go to "Add User from community".

I was clear on if that was the step you were trying, or if you were trying to re-sign up a user.

Centralized user registration is already built in. The blog signup page is also used for user signups.