I did a quilt like this for a baby shower and the organizer gave us all ONE piece of fabric we HAD to use in our block. Everything else we could choose from our stash.
For those who are creative on their own this will give them enough to go with. For others that need some 'support' you could have a sewing afternoon at the church and come with the one fabric, some scraps and some easy patterns. Then you could help those that want to contribute that don't have a clue what to do...
I would also EXPECT to square up and add border to the blocks to make them all the right size. If you plan on doing that at the beginning it won't be so frustrating when you get a bunch of different sizes. I've seen lots of quilts with these extra borders...and in a way they will tie the quilt together color wise to unify the 'scrappy' and 'unique' aspects of each block.