Managing blog members

A typical use case for our WPMU/BP site is a professor controlling access to a course related blog. Using More Privacy Options, the blog administrator (the professor) can use the setting: “Blog visible only to registered users of blog.” So far, so good.

However, we are authenticating against an external LDAP server and creating WPMU accounts on a user’s first login. We don’t want users with only WPMU accounts. So we have disabled the site wide option “Add New User” because it doesn’t distinguish between existing users and new users to the site. Now, there seems to be no way for a blog administrator (the professor) to add existing users (the students) to a blog. Is that correct?

Ideally, I’d users (typically students) to be able to request blog membership to the blog administrator (typically a professor). While this is implemented for groups, it doesn’t exist for blogs, right?

Can groups could be granted membership to a blog? If so, a teacher could manage a group to control access to a course blog. I don’t see this option. Am I missing something? Is there another plug-in I might use?

Thanks for any suggestions.