WordPress MU

I run a WordPress MU 2.9.2 installation for a university.
everybody with an email address that ends in @example.com can have as many blogs as he/she wishes.
now, some projects require that external users (with other email-adresses) work together in a blog.
I can add external users manually. but I want to restrict them to be editors or admins for only one blog.
and/or at least they must not have the right to create new blogs.

I have searched for a plugin, but did not find one.
I have looked in the database, but did not find a way to restrict this.

Is there a way to do this.
any hints would be much appreciated!
thanks!

cheers
kuene

Because it's built-in?

Under Site Admin -> Options, there's a field to restrict signups by email.

Site Admin *always* has the option to manually add new users in the backend, as well as add them specifically to whatever blog you like. Adding them to the system as a whole only gives them the ability to get to their profile page in the main blog. If they are assigned to a blog as their primary, then they see their profile there.

sorry. I thought that my post was clear.
>>Under Site Admin -> Options, there's a field to restrict signups by email.
yes I am aware of that. but this is not what I meant. I need sign up by email for the email addresses that end in @art-uni.edu.
my university is called art-uni.edu. every buddy can sign up that is part of the institution and has an email address @art-uni.edu. they can have as many blogs as they wish.

but some of the blogs need editors that are not part of the university and don't have an email address ending in @art-uni.edu.
so I can create this special user manually. but I do not want them to have the right to create blogs.

john@art-uni.edu (is part of the university) is admin of blog A
and he wants to work with sara@someotherdomain.com (is not part of the university) in a research project that uses blog A.
so I make a user for sara. but I want that sara can edit only blog A.
and I do not want sara to make new blogs. even if I restrict saras email domain now, as you suggested, she will still be able to make new blogs.
I want every member to create new blogs apart from sara.

I hope I was clear now. ;)
Is it possible to restrict only some specific users from creating new blogs?

what happens,if we append some conditions in wp-signup.php :
in function show_blog_form like:
`global $current_user;
if(substr($current_user->user_email,-12,12)!='@art-uni.edu'){
return error;
}
else{
...
..
show blog creation form;
...
...
}`
I didn't used it....just an idea..