Unfortunately you can't book reservations for guest passes (the half price ones I assume) but if they are going with a member each member can make 6 reservations for the same day. If not just take them to the gate and do it that way. We had reservations and they didn't even look at them but ticket sales were also open and people were allowed without a reservation.
https://zoo.sandiegozoo.org/reservations
We have booked 2day pass online tickets.we were holding the ticket confirmation number. After entering the park ticket vending machine was there.we had entertered the confirmation number without any space.
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