Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
TeamSnap - Sports Team, Club & League Management Software & Apps
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
CoachHippo - The ultimate membership management tool for managing your club, nonprofit and organisation. Collect payments online. - Get started for free!
Bigtent - BigTent is a web-based organization and communication portal for groups.
Simpler Sport - Save time managing your sports team