Guests and RSVPs
Through the personal link in their invitation. It opens a page with their name, your names, the date and venue. They say yes or no, adjust how many are coming, and can leave a note. No account, nothing to install, and they can change their answer later.
The link is the invitation. Each household's link is theirs alone, so the page already knows who they are.
They can come back to the same link and change their answer, right up until you turn RSVPs off.
The Guests tab shows households invited, guests confirmed, declined and awaiting a reply. When you open it, a banner tells you how many new responses have arrived since you last looked.