LinkedIn - LinkedIn is a business-oriented social networking service, mainly used for professional networking.
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
about.me - About.me lets you quickly build simple and visually elegant splash pages that points visitors to your content from around the web. Get started today.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Bigtent - BigTent is a web-based organization and communication portal for groups.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.