Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Patook - Platonic Friend Making and Community App
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Let's Try Sports! - Let's Try Sports is a website for finding sports activities in your local area: tennis...
Bigtent - BigTent is a web-based organization and communication portal for groups.