It could go so many ways!
Would participants collaborate on one track? That could decided by the level of interest, I guess.
I like the idea that people would work within a set number of bars and tempo, with the understanding that their track would overlap (wholly or partially) with another.
When submitting their track they might give an outline of key and/or chords, if relevant.
The mixer(s) would arrange the results. They wouldn’t edit tracks, aside from approximating levels and possibly fading in or out or using simple effects within the two bars either side of the 64-bar track.