Whenever someone hires me over the net to record something, I will usually have them send me a high bit mp3 llike 256 or 320 of the track. Then I will do my part, send them an mp3 back to them with my part mixed in. If they like it, then I will send them a wave of my part, so that they can put it back into their DAW and manipulate it.

The hardest thing I can foresee is how everyone will know what to play. I mean, will everybody work from just the original scratch track, or will one person at a time record, so that the next person who records will be able to hear what the first person did. Since it is just a "for fun" project, it probably doesn't make a lot of difference, but it would seem to me to be a bit hard to know how much or how little to play, if you don't know who and what else is going to be mixed in.

One thing is certain, there should be one person who receives all the files from all the players, who is compotent at editing and mixing, so that he can create the final project; taking out parts that turn out not to be necessary, properly EQ'ing, panning, assigning effects, etc.

And another important thing to note, do not trim your waves. Even if you only play for 30 seconds out of a 3 minute song; export a wave that is exactly as long as the original song file you received. Yes, it will take up a lot more bandwidth to upload and download, but it will be infinitely easier to re-compile so everything stays in the same place in the timecode.