Facebook Events - Facebook Events enables businesses to organize gatherings, manage invitations and send notifications and reminders to their audience.

Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.

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

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.