Do you mean 1vs1?
6vs6 would not work because you only can hold 6 players in a fireteam.
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Just make a lobby that can hold 12 people. I (or anybody in my fireteam) can invite you (or anybody else in your fireteam) to the lobby and go from there. If any team has less than 6 people, then someone can join up on that fireteam.
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If that was the limitation for the exclusion of private matches I would be pretty mad. If you could do private matches, but 3v3 I would be still very happy :D