Round robin, everyone makes a center square (or rectangle). Next person adds a 3 inch border all around, next person a 5inch then next a 6 inch, then a 7 then final 8 inch border all around. (sizes of borders can be altered as agreed upon) Be creative! expand on the center originals theme and colors! Dont just add a strip of fabric. here is one I had the pleasure of working on. you can see the center (she included the plain purple frame), the next persons border added the light green diamonds and pinky purple diamonds, almost like entwined ribbons, I did the next border by building upon the last border by expanding the lite purple and adding the Dark green lines shooting off the dk green units and adding the blue/green batik into the mix and used the "crowns" for my cornerstones. (I paper pieced.) The last border Julie built on what I had done. Using Angle play blocks she stayed with triangle theme and pulled off the darker purple from the previous to make the whole center just expand into a glorious huge work of art!
It still has a couple of more rounds to go. But you should be able to see how each additional border builds on the last..
This to me is an awesome example of a Round Robin.