If you backed Drift Stage on Kickstarter or through the Humble Store, you should be in the Backer user group and have access to the private Backers forum. If you backed at the "Tester" tier or above, you should also belong to the TestDrivers user group and have access to the Test Driver forum. Access is granted based on the e-mail address on the account you backed Drift Stage with, so make sure you're using that email address here (you can change it from your user preferences page). If you've just recently signed up for this site or backed the game, please allow at least 24 hours to receive access.
If you encounter any issues with getting access, send an email to firstname.lastname@example.org from the address on your Kickstarter / Humble account with your name and how much you contributed.
NOTE: This entire discussion forum is invite only for a short time. Very soon anyone will be able to join, but the Backers/Test Drivers forums will remain private and exclusively accessible by backers.