Another suggestion would be to buy some more plain fabric for the guests to sign on (with freezer paper backing) and incorporate the signatures into the back of the quilt. Then, on the front squares you could embroider the date, place, bride and groom's name - things like that. Or put some nice applique there. People just aren't used to signing their names on fabric, and someone is likely to make a mess of it. You wouldn't want them to mess up the lovely quilt top.