Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
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
Bigtent - BigTent is a web-based organization and communication portal for groups.