How to add an RSVP (arrival confirmation) link to the reminder text?

All you need to do is to add the "$rsvp_url$" place holder to your reminder formula. This will add a short link to the text message reminder. When the invitee will click the link he will be directed to an approval page.

After confirming arrival you will be able to see this in Un4gettable interface and as a  ✔ sign, in the invitee field in the relevant calendar event.