From Social Patterns
A group of users want to meet up spontaneously.
Use this to enable groups of people to meet up in either a planned way or spontaneously based on location.
Allow users to easily see where their network is located (with permission of course – see the Boyfriend Anti-Pattern) and send out a message or invite to meet up to those people.
Allow users to send out one message to a pre-defined group of people.
Consider the option of allowing users to send out one message to a group pulled together on the fly – based on proximity.
Using a combination of geo-location and text messaging users can easily pull together an impromptu gathering. Services like Dodgeball Foursquare and Loopt offer the ability to see yourself and those in your network plotted on a map.