"4 player co-op multiplayer
Join your friends in the all-out galactic war to take Earth back. The universe of Mass Effect expands with the addition of new co-op multiplayer missions, playable over Xbox Live, PSN and PC internet. 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. Success in multi-player will have a direct impact on the outcome of the single player campaign, giving players an alternative method of achieving ultimate victory against the greatest threat mankind – and the entire galaxy – has ever faced.
Mass Effect 3: Galaxy at War
As a bonus to the campaign, BioWare is introducing the Mass Effect 3: Galaxy at War system, a new way for players to manage and experience the galactic war from multiple fronts, including a new 4-player co-op mode. The key to saving the galaxy is the “Galactic Readiness” level, measured by Commander Shepard’s ability to apply
every possible asset – people, weapons, resources, armies, fleets – in the final battle against the Reapers. Players can impact their game’s Galactic Readiness level in multiple ways via the Mass Effect 3: Galaxy at War system, including multiplayer. Other platforms and interfaces will be announced in the coming months. It is important to note that the system is entirely optional and just another way players can have control over your game experience – it is still possible to achieve the optimal, complete ending of the game in Mass Effect 3 through single-player alone."
FAQ on the link below.
Can't say I'm particularly enthused about multiplayer in a series that doesn't need it.