They typically start in and work their way out. So, first they ask employees, maybe close friends/family of employees, then reach out to ACE and other friends of the park. If they need more people than that, they'll start to share the invite to a wider audience. I know other SF parks will post the casting invite to their Facebook, but I've never seen Great Adventure do that. You mostly have to know someone or be in a club that gets the invite.
I've attended several shoots for Zumanjaro, Holiday in the Park, and El Diablo (and have been invited to almost all since then). For Zumanjaro, only employees could ride due to not having state approval at that time. A bunch of ACE members rode Kingda Ka during the shoot (myself included).