WordPress Mu – Adding members to the blogs : how ?

Hello ! ??

Is it possible for the admins to add members to the blogs created with WordPress Mu ?

I’ve seen it’s possible to add a member who already belongs to the community, but this is not what I need. In fact, I need to add members, even if they don’t belong to the community…

If this is possible, can you tell me how ?

Thank you ! ??