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
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
SportEasy - SportEasy is an awesome app to communicate with your team and organize your games and practices...
Bigtent - BigTent is a web-based organization and communication portal for groups.