Missions are spawned into the free roam game. They are similar to what you could expect out of a typical single player mission. Not just cop killing like most coop gangs.
Example (I recommend watching on Youtube in 1080p):
The description on Youtube contains all the details you'd need if interested in joining.
Peace be damned.
Edited by Vintage88, 24 January 2012 - 06:48 AM.