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In the arena queue, it waits a second or two after there are 6 players before the players are sent to the room. This means that even more players after the 6 can join in that moment.
However, rather than the latecomers having to wait, the game doesn't send the 6 players who've been waiting the longest to the game. I can't really tell if the selection of 6 is just random or if there's a specific pattern though.