Officially, Microsoft has only been saying that Sumo Digital, Reagent, and Cloudgine are working on the game. Sumo is handling the campaign, whereas Reagent and Cloudgine are doing the multiplayer.
http://www.neogaf.com/forum/showthread.php?p=240288453#post240288453
But a few LinkedIn profiles reveal that there's another company involved: Crackdown 2 developer Ruffian Games is seemingly also working on the multiplayer part of the game, which is not shown at E3. To what extent remains to be seen.
Who is working on the game - We (Sumo) have been involved with the game from the start and we're principally handling the Campaign side of the game. It's a group effort though with Reagent and Cloudgine who are principally handling the competitive multiplayer. Some elements are shared, some are unique based on the gameplay needs.
http://www.neogaf.com/forum/showthread.php?p=240288453#post240288453
But a few LinkedIn profiles reveal that there's another company involved: Crackdown 2 developer Ruffian Games is seemingly also working on the multiplayer part of the game, which is not shown at E3. To what extent remains to be seen.