Many of the 300,000 users that are currently playing the Halo 3 beta are skillful modders who over the last few days have been examining the beta's code. They have found references to unannounced weapons, new vehicles and game types but probably their most precious finding is the fact that Halo 3 will support up to four players co-op on Xbox Live and System Link.
Scrolling through the list of on-screen text for party menus, one sentence revealed the following:
The game cannot start because all players are set to observer. Too many players for splitscreen. Only 2 players may play splitscreen coop on the same Xbox 360 console. The party size is too large to start the game. Up to 4 players may play coop on Xbox Live or System Link. Loading information from Xbox Live…
For more info use this link ; Team Xbox