First of all, you need to install Invitations app (included to UNA PowerApps).
After its installation, enter to the Settings tab there and turn on "Enable registration by invitation only" parameter. In this case when the visitors of your UNA site will click a "Register" link they will be informed about the registration by invitation only with the offer to fill a form to get an invitation link. In the tab "Requests" operator will see the lists of the new guest and may send invitations to all who looks good for his site. So every user gets an individual link and no sense to show it somewhere else except the letters.