Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Ticket19 - Sell tickets for your events with minimal transaction fees.
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
pretix - pretix helps you to sell tickets for your event in an easy way. It supports multi-lingual events and provides a wide range of features
Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.