Called the number as you suggested and learned that on the Flight Pass "My Bookings" page there are 2 red buttons. "BOOK FLIGHT" and "VIEW". Clicking on the "VIEW" takes one to a screen where the pass details are reviewed... cost, flights to date and at the bottom a spot to add the additional passenger.

Thanks for your help.

(BTW it was the 8 credit $1,260 "Central Escape Pass - Canada")