I use Drupal for one of my sites (
http://www.tigersfortomorrow.org) and they have a great feature that allows login into ANY drupal site using a special login format, e.g.
kiles@tigersfortomorrow.org. This allows me to login to any Drupal site on the planet without having to register again.
Im sure this does not answer your specific issues with group access but you may want to download Drupal just to see how thats implemented.
Im not Hawking Drupal - Mambo is my preference now. They just have some features I really like. Especially the Book feature that links multiple content pages together in a specific order (a book) with the proper navigation. To do that with Mambo the document must all be on ONE page with page navaigation added. This is no good if you have multiple editors working on different Book sections. Of course - you could just create a Content Section in Mambo but then you have to rely on the Breadcrumb to navigate to the next page. Not very intutitive to Non- Mambers. Would really like to see a mambot that provides a Next and Back link within the content section.