What if some guests, like a lola or ninong, don’t use smartphones?

They don’t need to. You can add every guest to the list yourself and record their RSVP for them, so your headcount and seat plan still include everyone. At the door, your coordinator finds a guest by typing their name — there is no QR they have to scan. Guests who do use a phone need no app and no account: the invitation, the RSVP and the photo camera all open in the browser from a link or a QR code.