or, everyone is given the same blank piece of score paper (or 4-5 pages) and the same melody to start off (or motive). we each go and create something and then compare later. the form would be announced (prelude, fugue, sonata, fantasy, whatever) before everyone goes to do their thing. or, perhaps everyone is given a different form.
i find working alone most satisfying to the result and would become highly frustrated by others dabbling on what i did.