Keep students in groups of 5 or less. More than this starts becoming unruly. A group of 4 or 5 can generally keep their own order, though 'less students is always more' as with every other aspect of teaching. Some students will love composing alone and this can be very rewarding for them. Others will prefer 2's or 3's.
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.