Yep, we actually do support that now on signing links. Any field on the contact page of the quote checkout is also required when e-signing a document, including booking custom fields flagged for checkout: https://www.ownerrez.com/support/articles/capture-additional-fields-on-checkout-quote-acceptance -- we sort of stealth added that a month or two ago and haven't fully announced it.
We don't have a dedicated guest list field, but you can either do a rich text custom field with a description that asks for name and age of each guest one per line, or you can add multiple fields, one for each name.
