If you want to create a new developer account, simply click on the Free Sign up button on this link and provide a new email address, follow the instruction to complete your new account.
The https://service.devtest.ringcentral.com site is for you to set up your account such as adding new users (extensions), assign phone numbers to a user, read call log, change call handling configs etc.
Each sandbox account allows you to add up to 4 users.
Actually we already have bought the ringcentral premium account. But we just dont know the credentials for the developer account, is there anyway the admin can create a new development account for us? to connect with the live account?