Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Mobilizon - A free and federated tool to get our events off Facebook
Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
Bigtent - BigTent is a web-based organization and communication portal for groups.