Collaboration is a wonderful thing - set up some ground rules and keep things democratic. Don't force groups that don't want to work together. This is a creative venture and working with kids they don't want to work with is a hard way for kids to feel free and uninhibited. This is one of the biggest mistakes I have seen teachers make. You can put unlike kids together on a softball team, but it doesn't work as well in an art situation unless the kids are very mature.
Smaller groups can opt to perform their pieces.
I usually have them perform 'extra' parts along with all they have created. This takes much of the pressure off of trying to replicate what they improvised and edited with the technology. This is advanced stuff here, but I generally treat it as a common place thing and see how students react to it. Many will just go off and learn their melodies by ear. This level of transcription is usually reserved for college music courses, but by treating it casually, many of my students will just do it.