For my daughter's wedding quilt, I had plain fabric squares ironed onto freezer paper for the guests to sign with Pigma pens. Afterwards I constructed the quilt using batiks, fabric from the bridesmaids' dresses, and those squares. It would be too hard to have people write on a finished quilt (in my opinion). Batiks aren't a good fabric for writing on, I don't think. Another option would be to send out squares ahead of time, gather them, and make the quilt before the party.