Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Glympse - Glympse is the easy way to safely share your location in realtime. No sign-up needed.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Zenly - Zenly is a GPS tracker to locate your friends in realtime.
Bigtent - BigTent is a web-based organization and communication portal for groups.
OwnTracks - Cross-platform location sharing