The easiest would be to copy the "Register Here" link from the Opt-in Invitation visible in one of the traveler's trips from the Trip Center tool and paste it into an email to the traveler. The traveler could then use this link to register.
In the Trip details page, you'll find the Opt-in Invitation on the right-hand side:
Click on the traveler's email and you will be brought to a new page that will show you the exact email that was sent to the traveler with the "Register Here" link that you can send to them.
How can you tell if the traveler has responded to the Opt-in Invitation?