I don't know - I was able share a link and RSVP without an account. AFAIK I enabled every option other than the shared album.
It does prefer contacting via email, so it did an email verification via mailed PIN, and then attached that email to the guest list from the link.
I originally created the event using my own Apple account which definitely has iCloud+. So how do I create an event that someone without an Apple account can RSVP to?