"Over the past few weeks we've been working with Microsoft to lay plans for the future of State of Decay, including future Title Updates, DLC, and possible sequels. As part of that process we've also been thoroughly evaluating the possibility of adding co-op multiplayer to State of Decay. While there are great things in store for State of Decay, unfortunately multiplayer is not one of them."
Source and full article: http://www.videogame...-op_update.html
Have yo say I'm pretty sad about this, I've been holding out hoping they would add it. Lets hope they do a new version soon with Co-Op from the start.