Add each of your guests to the guestlist and send them their personalised invitation link via their preferred social app (eg. Instagram / Signal / SMS / Element / WhatsApp).
When they follow their link they can click a button to confirm their attendence (RSVP) - without signing up. Though of course - they might sign up to host their next event on Gatho!
Write your invitation message in a Matrix chat, and ask your guests to react with a thumbs up or down. The Gatho bot will add those guests and sync their status to the Gatho event page.