Gears of War 3 - Campaign

You can join other people's Standard or Arcade Campaigns. Standard is basically the same as the single player except with up to three other players. Arcade mode introduces a scoring system and mutators into the mix. As you get kills, you gain points and if you don't get hit you gain a multiplier for points you gain. You can also use mutators for things like Big Head mode.

You should be warned that in both modes, the more people there are, the tougher the enemies become. They will start to use more advanced tactics, they will have more HP, and Drudges will be more likely to mutate.