Today, almost all young folks have access to PCs and scanners. Why not have them e-mail their scanned signatures to you? It's a lot easier than making and mailing fabric squares, quicker than snail mail, and you won't waste fabric. Then, you can resize (if needed) and print the signatures in whatever colors you want onto printer fabric sheets (purchased or made yourself, using Bubble Jet). You can even add borders and other artwork, if you like.
I'm glad you asked the question, because it's giving me some ideas for a family siggie quilt.
I collected fabric signatures at my retirement party and at the installation celebration for a new priest at our church. So, I used the treated fabric and freezer paper method with pigma pens. I was lucky enough to have volunteers man a table to collect the signatures at the entrance to those parties.
Good luck,
Nancy