We had 2 parties of 4, 4 on Steam and a player with an Xbox and 4 controllers.
We had a hell of a time being able to start until everyone was friends with each other on Xbox.com. We couldn't start the search or someone would drop.
We had every region checked and if we got paired with randoms we waited the match out to queue again. Once we started connecting to each other it was easy, just have to wait for the right mode to get selected. Social, FFA, H:Reach, Asset Denial.
We did collect 10 skulls and turn in all at once.