I have done several things to make it work as we wanted.
The AD-authentication plugin creates the user, but not a site. Thus registered users are allowed to create new sites. As we wanted to control the URL and only allow users to create 1 site I had to change the code in wp-signup.php and wp-admin/includes/ms.php.
If you are interested in the details I can post them, but basically my code forces the URL to be host/username. Since the users cannot change this they will get an error message if they try to create another blog. The change in the ms.php is just commenting out the line printing the “create another site”-link.
Still searching for some documentation related to my setup….
Cheers,
Kristian