In our guild with over 70 members, only 6-7 people sign up to join a round robin group. We each made a central block of our choice and included whatever background and other fabric we wanted used. We also included a note about size, color, and style preferences. People could add fabric if they wanted, but we kept the rounds at no more than 6" high. The results of this activity were fabulous. You could limit the groups to 4 or 5 if more people wanted to join in. You could also do a "row robin" so that each row was left loose, to be sewn together by the owner. When I've participated in these, a length and height of each row was decided ahead of time.