Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Groups.io - Email and web based discussion group software, includes archiving, emails, wiki, polls, file...
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
Bigtent - BigTent is a web-based organization and communication portal for groups.