Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
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.
Ticket Tribe - Sell tickets online with no fees per ticket
Bigtent - BigTent is a web-based organization and communication portal for groups.