This support article applies to RSVPify 1.0 and 2.0.
Question: Instead of just allowing a guest to bring 1 guest (a +1), is it possible for me to allow a guest to bring multiple other guests (a +X)?
Answer: While on our 2.0 roadmap, at this time it's not possible to allow a guest to bring anything but a +1.
Setting up a guest list if you don't know all of the names in a given party: With our 2.0 release, however, you can add a party with the name of your primary guest -- and then add a guest for each person that you'd like to allow the primary guest to bring.
Make the first name something like 'Guest First Name' and last name 'Guest Last Name'. Then, when the primary guest goes to RSVP, they can choose Accept or Decline for each of the guest's spots.
Be sure to set 'Prevent guests from editing pre-set names' to 'Off' (setting in your Guest List tab) so that the primary guest is able to add names of any guests they plan to bring.