drupal7

The successor of Organic Groups modules is just called "group" (http://drupal.org/project/group).

The main new features are group roles and group permissions. Longly awaited functionality if you ask me. Can't wait to play with it.
Per group you can create roles, and assign permissions to those roles. And people can have different roles for each group.

Nice video screencast: http://www.youtube.com/watch?v=vDtFJyfGEaE