I went to a bridal shower where we copied Bible verses onto fabric squares that were later made into a quilt that was presented to the bride on her wedding day. We used fine tip Sharpies. Worked great for most people. Of course, you always have that one perfectionist in the bunch that wants to re-copy it four or five times until it's up to her standards.
If they're going to sign the actual quilt, I would make sure to show them quickly first & emphasize that they don't want to leave the pen in one space for too long or it will bleed, potentially straight through the quilt (happened to my friend Sarah's quilt in high school). Keep some hairspray on hand for ink emergencies. Aerosol works best. And realize that like anything else with quilting, that there is the possibility of a boo-boo but ... applique covers over a multitude of mistakes.