Organic Groups is one of the most powerful social networking modules for Drupal. It allows users to maintain groups. The managers of these groups can control who can join, or post or view content in their groups.
Essentially, these groups can act as microsites inside the site; they have their own user list, content, and access control. For example, a football site might want to maintain groups for football teams, and post news and announcements that are relevant to the team in the team's group. The players would then join the OG group of their team. When players visit the group's page, they would see only that team's content (nodes, technically).