Think about the last form you abandoned. It probably asked something you did not know, or something you saw no reason to give. A reply page is no different. Every field you add is a small chance the guest pauses, gets busy, and never comes back. A guest RSVP page should read like a kind question, not a registration.
The design goal is honesty at low effort. You want guests to tell you the truth about whether they are coming, who is with them, and what they can eat, in a way that takes seconds and never makes declining feel rude. The fields and the wording are how you get there.