You can already tell before the match begins who is part of a fireteam and how large the team is. Hit start and go the Roster. To the right of the Gamertag you'll see a black bar if it's a team of 2 or more, the larger the bar is, the larger the team is. I usually run with a wingman, and I do notice often we'll be placed in a game with another wingman team on the opposite team. I've seen in several occasions, multiple wingman teams in one game and once the entire match was 6 wingman teams... it was pretty cool.