A link that opens straight onto the invitation. No sign-in, no app, no password.
They see the title, your name, the day and time, where it is, your photo if you added one, and your note. Underneath: a map and Directions if you gave an address, and the who's-coming list if you left that on.
The answer is two buttons, Yes and No, sitting where a thumb already is. Not sure yet is a quiet third option, and it comes back to them later rather than pestering you.
If they are bringing people, they say how many on the same screen — chips up to six, a stepper beyond that, capped at whatever you allowed.
The page loads nothing from anyone else. No advertising, no tracking scripts, and no cookie at all until they answer.