Do you need the full contact details of the guest, or just a "+1"? In the latter case, you could simply create a custom field for that. If you want to get the full contact details, do you want to store those as a separate Lead record? In that case, you could create a second form for registering the guest(s). Set that page as the thank you page for the first form. Also make sure to remove the Munchkin code from the second page, so any web activity continues to be linked to the primary user's email address.