Two suggestions: 1.) Pre-wash your fabrics without adding any fabric softener. This removes any sizing on the fabric and the markers will penetrate the fibers of the fabric better. (I'm assuming the fabric is 100% cotton. Natural fibers dye better.) 2.) Just have the blocks available for signing, not a finished quilt. That way, if they mess up when signing or write something unacceptable, it is not already a part of the quilt.
We had blocks available for guest to sign at my daughter's wedding last year, made with scraps from clothes I had made for her as a child. It was truly a memory quilt for them!