The reply half, and the privacy of it
Answers need a deadline, and the page shows one, in the guests' view rather than in a message that gets quoted and misremembered. Until you publish, the draft is invisible to everyone, so the invitation you perfect on Tuesday does not leak on Monday. If the gathering is private, a password guards the page and you hand it only to invited guests.
Then the message goes out. One line, one link, into the chats you were messaging anyway. The chat keeps being a chat, jokes and all, while date, venue, menu and every yes land quietly on one list that has no sense of humour at all, which is exactly what you want from it.