I would think that, while adventuring, you'll be paired with another group with as many players that are in your group. So if you're adventuring on Mars with 3 other people, when you meet another group they'll have 4 people too.
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All I know is that I expect to have a giant battle defending the wall with like 30 people by the end of the game. Warlocks casting spells, hunters sniping and running around, and Titans heaving grenades into crowds and mowing aliens down with light machines guns, etc. Oh yeah.