Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Wiggio - Wiggio is an online toolkit that makes it easy to work in groups. With Wiggio you can:
Groups.io - Email and web based discussion group software, includes archiving, emails, wiki, polls, file...
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Allt.in - One of the best work management tool your company needs.