Spond - The easiest way to organize team sports or other type of group activities
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Fullcourt - The social network for basketball players
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
TeamSnap - Sports Team, Club & League Management Software & Apps
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.