It's so easy to mess up an already-made quilt, I would recommend having the guests sign on previously prepared fabric, ironed onto freezer paper for stability. Use some blue masking tape to provide boundaries around the area where you want them to sign. Then if someone makes a mistake they can try again, and if someone leaves a comment that you don't want in the finished quilt, you don't have to include it. I think pigma pens would work well for this.