There's a couple of options to achieve this. The easiest would be to copy the "Register Here" link from the opt-in invitation visible in one of the traveler's itineraries in the Agent Advantage console and paste it into an email to the traveler. The traveler could then use this link to register.
When on the view itinerary screen, you can use the "Edit Itinerary" tool to remove the email address from the PNR by clicking on the red 'x' next to the email address. The email address will appear in two locations, the Email Address and the EMS Delivery Points sections. After you've removed the email address from these locations you'll need to hit save.
To generate another Opt-in invitation you'll just need to put the email address back in those fields and hit save. The Opt-in message may not be displayed right away in the console, but another one will be sent at this time. Important note: Please place the PNR back on queue to FlightStats once you've included the traveler's email address in the PNR.