Bungie has now posted an article detailing everything you could want to know about the feature. So, without further ado, the nitty-gritty:
* 4-player co-op via system link or Xbox Live
* Splitscreen limited to 2 players, but 2 more can be accommodated via system link or Live
* Co-op is managed in its own lobby, similar to other modes
* Players can still save films of co-op sessions
* Bungie will be adding some incentive for multiple play-throughs
Three elites will serve as the Master Chief's co-op partners. Player two, naturally, will play as the Arbiter. The other two are newly created Elites with their own backstories. It seems that since the Schism of the Covenant in Halo 2, humanity finds itself with a growing segment of Elite sympathizers.