Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Swarm - Swarm lets you earn prizes and compete with friends based on the places you go.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Arc App - AI powered location tracker
Bigtent - BigTent is a web-based organization and communication portal for groups.
Foursquare - Foursquare is all about helping you find new ways to explore your city.