BioWare has confirmed that the third and final chapter of Commander Shepherd’s story, Mass Effect 3, will ship with a four player co-op mode, putting an end to fan speculation.
The new four player co-op missions will be seperate to the single player campaign and will not be "affected" by savegames of previous Mass Effect adventures.
"Players can choose from a variety of classes and races, form an elite Special Forces squad, and combine their weapons, powers and abilities to devastating effect as they fight together to liberate key territories from enemy control."
The co-op will tie into your Single player experience, although BioWare has kept a tight lid on how, only revealing that it will be an "an alternative method" of achieving success in the main storyline.
BioWare says that the co-op is one part of a larger part of the game they called "Galaxy at War," something else that BioWare is keeping a tight lid on, although it is said that it is an optional system that uses many of the game’s components to "augment" Shepherd’s "Galactic Rediness level."