I did this last November. I found the information by googling it. There's a phone number that you call 30 days from your appointed day and make a time. If you prepay for the photopass, the session costs and additional $25 (at the time, I'm not sure if it went up since then), otherwise it costs $50. You just have to bring proof of prepaid photopass. Its not really a professional photo shoot, its just a private appointment with the Photopass people.
The sessions last about 45 minutes and you take pictures around the Grand Floridian. It was nice to have private time for pics. Make sure you get both family pictures and individual ones takes. It was a nice momento from our time there. We took pictures in the lobby, outside by the pool, by the beach and inside near the winding stairs.
I just put all the pictures on my Photopass and ordered them when I got back. I ended up with about 75 pictures by the end of it.
Sorry I can't remember the number to call. Hope this helps a bit.