plug all your friends' instruments/mics into a mixer and then plug the output of that mixer into an audio interface hooked up to a computer to record it into a DAW. or just plop a handheld recorder in the middle of the room and let it ride. or just huddle really close around someone's phone recording on voice memos. depends how much gear you have. as far as rules, i'd say treat it like improv theater/comedy. don't say no to any idea, just "yes, and-"