There will be no co-op or multiplayer in Marvel's Guardians of the Galaxy game. Square Enix handed this project off to Eidos Montreal, who stated that they did not intend to make it co-op or multiplayer.
This was revealed shortly after the game was announced. Guardians of the Galaxy's Senior Creative Director, Jean-François Dugas, said in an interview with IGN:
"We started to discuss different opportunities like multiplayer or co-op. But we were really looking at what Guardians of the Galaxy is, and it's all about those characters. It's all about colorful personalities clashing with each other and sometimes getting along. It's a mix of all these things, their qualities, their flaws. And we thought, 'It would be freaking awesome if you could be one of the Guardians and put you at the center of that to be surrounded by those characters.' And this is how it came to be."
It's a bit of a heartbreak for many fans of the game, but the quote above explains its reasoning. Marvel's Guardians of the Galaxy appear to be taking a different approach from Marvel's Avengers.
For more info on Marvel's Guardians of the Galaxy Game, check out Can I play as the other Guardians in Guardians of the Galaxy? on Pro Game Guides!