The ideal way to work this out would be to start with a concept. A style. A tempo. An intended feeling or specific subject.
One person could start by submitting a simple riff or backing track. Others would be free to download and add to the track as they desire BUTI think it would be fair and desirable to allow the person who started the concept to audition parts and complete the final product using the parts he/she feels best suit the concept.
Ideally each collaboration should be submitted as a discussion. Further additions and submissions should be added to the discussion or by private message in accordance with the discretion of the conceiver.
Tracks should be submitted equal to or greater than the full length of the backing or starting riff. The submission should start at the same time as the original riff and should include any dead space that was present in the original concept. This will ensure that syncing tracks is simple and consistent.
These are just suggestions however so, feel free to work in a manner that suits you best. It should be noted though that these suggestions have worked well on other forums for forum jams and the like.
Please HAVE FUN with this. Take it as seriously as you like but it would be awesome if we could avoid any sort of flaming or any reason for resentment on anyone's part.