I have done this for showers, weddings, retirements, family reunions, graduations. It's always nerve racking, because of the concerns your have indicated. Prism's suggestion is excellent if you are having the signatures obtained at the wedding. Downfall is it's best to have someone babysit the table to answer questions and try to catch mistakes before they happen., guests will still be confused as to what to do. We quilters think that everyone understands what we do and most people don't know which side of the fabric is up!
Be sure to wash the fabric of the factory sizing, which impedes the ink from setting into the fabric. And most important, purchase good ink pens, not cheap ones, they leak, splatter, and later fade.
I also found it best to have a fabric sample available to try writing before they put pen to your leaves. Signing fabric is very different that paper, and most people will press to hard and cause the ink to bleed. If they try a practice signature they are less likely to make a mistake. I just iron a large piece of muslin onto freezer paper and let them practice.
Good luck, it's a wonderful gift for your Son & wife. My daughter just celebrated her 19th anniversary, and her shower was the first signature quilt I made. It still hangs in her bedroom.
Pat