Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
The Groups Place - The Groups Place is a new alternative to Meetup.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.