If you’re not sure whether you are already registered for the site, please try to reset your password.
You’ll see that you are asked to provide your e-mail address or username. If you are already registered, the site will recognise this and send you an e-mail so that you can reset your password. If you don’t want to reset your password, you can just ignore the e-mail.
If you are not registered on the site, your e-mail and username will not be recognised. You can then register here.