Hi, Don. Our free Event Tickets (ET) plugin provides RSVP functionality but our Event Tickets Plus (ET+) upgrade is required to check-in these users, such as with a QR code on each of their tickets (to be scanned at the door to confirm whom showed and whom didn’t).
There’s also a “Check In” button next to each attendee in the wp-admin Attendee Report screen… so someone could RSVP to your event and it would send them the free ticket (just the way it is but the email could be disabled), and then you could have your own computer there at the check-in desk and just click the Check In button without them bringing a ticket.
So I’d give the free Event Tickets plugin a run-through. Then, let me know if you have any follow-up questions.