Multiplayer co-op is separate from the main game:
http://masseffect.bioware.com/me3/game/multiplayer
Which characters can I play in co-op multiplayer? Can I play as Commander Shepard?
Commander Shepard's part in the war will take place in the single-player campaign, as will that of other beloved characters in the franchise such as Garrus, Ashley, and Liara...these characters do not appear in the multiplayer missions.
In multiplayer, players will create custom characters to fight on different and unique fronts in the war. This will include the ability to play as favorites like Turians, Krogans, Asari and more... each with their own unique set of abilities.
What if I don't like multiplayer - will my experience be negatively impacted?
Mass Effect 3 is a complete, standalone game that will deliver a satisfying story experience, even if you choose not to try multiplayer.
The Mass Effect 3: Galaxy at War system and all of the individual components are meant to complement that amazing game and can be enjoyed on their own or as part of the Galaxy at War experience.
How did developing multiplayer impact the single player game?
BioWare is dedicated and focused on delivering an engaging, fun, and action-packed experience for Mass Effect 3, one that lives up the BioWare standard. To reach that level of quality, BioWare has a dedicated studio in Montreal that is home to designers, programmers, engineers, and other developers. Both studios work together as partners, lead by the core Mass Effect team, unified in a single vision. Under the direction of Casey Hudson and other team veterans, both studios make contributions to both the single player and multiplayer modes in Mass Effect 3. Rest assured that no compromises were made to either of these modes in the development of Mass Effect 3.