Maybe the backer forum was the comment section on the Kickstarter page
Joke aside, I don't think opening a backer forum now would be really useful. Maybe when we'll get a more in detail trailer or as other people mentioned, the demo, then maybe would it be better to open the backer-only forum.
By the way, how could they make a backer only forum? You'd receive a code to enter during the signing in process I imagine?