The game will actually feature both synchronous and asynchronous co-op for the campaign. What this means is that you will explicitly start up a campaign with your friend, each of you will get your own squad of Rangers to control and kit out as you like, and you're free to stick together or wander about independently. You can even continue playing the game if your friend isn't online, and vice versa. The actions you or your friend take within the world while the other is offline will still affect it, thus you could log in to some startling changes.