I like doing collaborations as well. I have collaborated with a few people on this forum and it is fun to do. My instrument is Piano/Keyboard/Vocals etc . . . and I use a Yamaha Tyros synth for my midi sounds.
Most people usually collaborate via e-mail or pm's here on this forum. I have seen other forums do their collaboration in their forum posts. Example; I am a guitarist looking for Drums, Bass, Sax or Piano etc . . then they post an MP3 with their part recorded with some basic bed tracks. They put the song info in the post as well, Tempo, Key and a request of which measures/bars they want for a lead etc . . .
The drummer d/l's the file, records his/her part and uploads their MP3 for the next musician. Most people are usually looking for a certain part they would like in their song. When you start using 3 or 4 people in a collaboration, the process takes much longer due to commitments in our daily lives, not counting a "do over."
But, if you want your song polished with a killer Sax lead etc, collaborations are the way to go, IMO. They are both fun and rewarding.
I like to collaborate and open to assist any way I can here. If you don't like how my part(s) come out, use that "Erase Trax" button.

Trax