It's my understanding that the "Stay & Play", "5% cashback" and The Guestbook are all part of the same deal. We had booked "Stay & Play" and/or "get 5% cashback" by booking directly with the resort website several times, so yes, we already had Guestbook accounts. When I first booked one of those "get 5% cashback" trips, I asked somewhere (here? this forum? maybe?) how we get that 5% back and someone had told me to register with The Guestbook.
We always book direct via Premier website. Then submit confirmation to the Guestbook. So we get 5% off the Premier rate.