There was a closed door demo (described as a theater presentation, journalists didn't get to play) where it was shown that not only is the entire map open all the time, but you can also split up over it for missions.
There are setup missions for the main missions (an example given is acquiring a helicopter), and each player can do them individually, but call for backup of things go south.
They're also including AI squadmates for when 4 people can't get together.