Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Invision Community - Invision Community is a flexible, integrated set of apps for building communities on the web.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.
Mobilize - Mobilize is a scalable messaging app built for large groups.