Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Galette - Galette is a membership management web application towards non profit organizations.
Cvent - Cvent's event management software provides event planners with a complete solution to increase event attendance and decrease event costs.
Bigtent - BigTent is a web-based organization and communication portal for groups.
EventMobi - EventMobi is an interactive mobile event app, audience response and engagement platform for conferences and trade shows.