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The loot doesn't scale with # of players, so you could just give them their starting equipment plus any loot you've saved for them. The XP mostly goes to all players, so you could just keep track of it and give them that much XP when they join.
That is pretty much what I planned on doing if there were no better systems. I was just going to give them all non-minion XP and save as much gear as I could.