If you are unable to sign in the first time after you join it is likely there is an issue with the username you were issued.
Check your email for a welcome email from the site you joined. It will show your correct username to log in with. If the username on your welcome email does not match the username on your billing receipt (Epoch, Rocketgate) you must use the username on your welcome email to login with.
Look for an email from the site you signed up with and use the info in the email to login:
If you can't find your welcome email, check this article here before contacting us, it'll save you time and frustration: I didn't receive my welcome email