How do I create a Discussion Forum?
I think you misunderstood the purpose of the Discussion forums - they are intended to be open to anybody who wants to join in, and be long lasting (kind of like the Community Chat forum). If you're just looking to hammer out the details of a game with the potential participants, then a private board is the way to go.
Then, once you're happy with the way the game concept is shaping up, you can archive the discussion threads for future reference, change the title of the board, and use it for playing the game.