"Use parachutes to take part in Multiplayer BASE Jumping throughout Liberty City. Find the 9 BASE jump marker locations scattered throughout the city in Free Mode and hit Right Button to warp you (and anyone else with you in Multiplayer) to the top of the building. Clean, successful and well-executed BASE skydiving jumps will be tracked by time and other statistics that you'll be able track and compare with Friends at Rockstar Games Social Club."
Additionally, more multiplayer details have been given on Race and GTA Race modes, with 3 new screenshots:
Link: TBOGT multiplayer