Hmmmm, the last time I took part in something like this - wala pang Internet nun! Back in the BBS days (this would be circa 1995), I used to moderate a forum called P-Audio on the local Fidonet network, and we had a project called "Midi JAM" where we'd add parts to a General Midi file based on guidelines. I think it would start with a basic drum/bass/piano track and as the file was passed around, people would add their parts and solos by MIDI, or reprogram the bass and drums, etc. One guy I remember who took part in this was Vic Icasas, who's now with HIT Productions. Vic was still a DLSU college student then.
Wala pang Internet email noon, so we used Fidonet's quaint mailing protocol (Fidomail) to send the Midi files to each other. And our modems were just 2400 bps modems! But we had a lot of fun and learned a lot - it was really like a jam except it wasn't in real time and we weren't in the same room.
With today's tech, you can of course come up with better sounding tracks. But the basic premise of collaboration remains.
Apart from playing original parts, I've also heard of people who sample rock/pop songs (obviously this is without copyright permission) and create "mashups". This is often a collaborative effort. The best ones I've heard combine Madonna and the Sex Pistols together - the result is a punk version of 'Ray of Light'. There's also a mashup of Blondie's "Rapture" with the Doors "Riders on the Storm", so it's like a funky Jim Morisson song. They are very good!! Ang galing ng editing, so it's also a cool learning tool for this.
Speaking of this, I'd like to ask someone out there to add a jazzy acoustic piano solo to the PhilMusic Podcast theme (it's tentatively called PM-Funk). This is the funky theme with the wah-wah guitar that starts and ends the podcast. I can email you the mono track and it would be nice if you can send me back a stereo file with the piano solo on another channel. I can mix it down to mono later (the podcast is distributed as a mono mp3 anyway). Any takers??