If you log into the forums using your email/password, you do not have to create a Derby account. You can Sign In to the Derby by using your forum login
If you log into the forums using a Twitter or Facebook account, you need to create a local Derby account to play. You must register with the email address that you log into the forums with. You will only have to do this once. After creating your Derby account, you will log in with the email and password you entered here.