I'm out, my guitar will be sold in 28 minutes.

This sounds interesting. I wouldn't mind doing some drums, although I could possibly do bass/guitar/keyboard.

I kinda confused then, does everyone get to hear what the others have done before adding their own or not?

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.

A Wiki style song writing tool? Like it.

Mine was a solo effort. 24 hours none-stop of writing and recording 20 minutes worth of music. Damn good fun it was as well, if a bit tiring!

Saying that, I'll happily contribute to this! Keyboard is my strongest instrument, although I could also add bass or (keyboard played) drums.

I'd play ukulele on it. I'm not very good at all at this kind of thing but I do have a reduced sense of shame.

No, it's a catastrophe!