I once wrote a song a bit like that, it was called 3 minutes of chaos and just had various drum loops, keyboard demos, me playing Bontempi instruments and sound bites from Spaceballs all layered over each other.
The easiest way to make sure it all sounds okay together would be to do it in turns, with one person recording something and then giving that to the next person to record over the top of. The last person would have pretty much a whole song to add their bit onto.
That's kind of the point, though. Everyone does their own thing, and then the listener has the choice of which one or ones to play at any point during the song.
No. There'd be a reference track (drums or something) as a starting point, but after that the players are on their own to do whatever, so long as the track is a fixed length. Then there'd be an interface that lets the listener play all the tracks at the same time, or just play some of them, or play them all but have the individual tracks fade in or out at points that the listener defines.