It's been unclear exactly how friends can cooperate in the new Sim City, but we did know there was going to be a strong social aspect to the game. Now the latest developer diary for the game has revealed a great deal just how cooperative the reboot of the classic series is going to be - and frankly - it's incredibly impressive.
Up to 16 players are able to join a region in Sim City and in each region there are areas to claim to build your own city. Cities within a region join a network of resources that all effect each other. In the video we see how one city runs low on power, so a friend's city can focus on building a power infrastructure and it'll automatically sell the extra power to where it's needed. Don't have enough firemen? They can be sent in from different cities. Everything that happens within a region effects the entire region and can benefit it.
In summary, it looks like EA has nailed a co-op city build sim.
Sim City is going to be available on the PC in March of 2013.