There should be an option to have a private fireteam that random players cannot join. Matchmaking is good for some instances, but it can be a henderance for others. An option to block or lock the pre made fire team would make a world of difference.
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It's next to the "Invite Party" it will say open to friends and if you click on it it will bring up a list of options. They should be "Public, open to friends, invite only, and closed. Us on 360 have it so you most likely have it.