Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
attending.io - Tiny event pages to ask people one question – are you attending? Twitter login.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Bigtent - BigTent is a web-based organization and communication portal for groups.
Alf.io - Looking for a solid open source alternative to EventBrite or MeetUp? Discover Alf.io! Keep control your event and your data with Alf.io. Get started today!
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.