Group

Project page: https://drupal.org/project/group.

The Group module allows you to create arbitrary collections of your content and users on your site and grant access control permissions on those collections.

All of the functionality this module has to offer is based on custom entities, opening up the door to various extensions or alterations that the user sees fit. The relationship between a group and its content or users is also an entity, allowing you to easily add metadata to that relationship.

One of Group's goals is to be as user-friendly as possible: configuring scenarios involving per-group behaviour, access permissions, subgroups, ... should all be possible through the administration interface, without the need for any custom code.