Having a "Signature Quilt" is a double edged sword. Lovely to look at; way too easy to mess up. I was at 50th Anniversary party where they had a completed (quilted) quilt. It was impossible to write on. Who knows if any of the ink is colorfast, washable, etc? People signed all over places that were not meant for signing, despite very explicit written directions, which no one took the time to read. More than one person used a felt tip pen that bled through to the back, and smeared all over the front. IMHO, they took a beautiful quilt and ruined it. People will be drinking, excited, NOT focused on the quilt, trust me. If you don't control the way they write the signatures, they could end up as a mess, and that's the last thing you need. I vote for giving them small sheets of white fabric (8 1/2 X 11, as a guess but could be any size), back the fabric with either wax paper or a thin, fusible interface (to make stiffer and easier to sign on), then drawn on the lines with a pen that will wash out, and have the piece signed with a pen that will go through the wash. For best results, make a visual sample of this, signed exactly the way you want your signatures to look, and display it prominently at the signing table. People can usually follow an example, or visual instructions if it's quick and clear. Then use those signatures on the back.
On the front you could add wedding date and names: of the Bride and Groom, and perhaps the wedding party, or part of it. You could do a little "family tree" of each side. You could add blocks to fill in white space. The choices are limitless.
Just a suggestion. I hope it turns out great! And BTW, after the wedding, get a long armer to quilt the quilt -- someone who can do lots of beautiful quilting for this heirloom. If you try to do it yourself, you will be pulling your hair out because you'll want it "perfect", and perfect quilting isn't in your skill set yet. So give yourself a break!