Following on from this discussion:
I had an idea that may or not be feasible. Would it be possible to create an email for each membership level that would be sent to new members when they sign up suggesting groups they could join which would use some kind of token that would autojoin them to groups that they clicked? Or maybe a form in the email that when submitted could add them to several groups at a time using the same idea but via radio buttons etc.
The email plugins and group email plugins all exist at wpmu but would be a great idea to implement some form of integration with them all.