Steam

when you play a game that multiple people have, you can choose which copy is being used. The owner of that copy and the one playing get banned

Best guess? Whichever account gave account 6 permission to play their game.

Either account 1, 2, 3, 4 or 5 will be the user that gives 6 the permission to play their game, so it follows they're the one that (I'm assuming) will get banned also. It's a good question you raise and I'd be interested to know for sure myself.

They send their enforcement squad to all houses involved.