Bhuvan is now using "Central Authentication Service(CAS)" to enable Single Sign-On(SSO), you can use the same log-in credential if you are already registered with Bhuvan.
Registration is optional in Bhuvan. However, some features require registration. Registered users are having privilege to share the data, collaborate with other bhuvanites, Forum etc. If not registered, it only takes a few moments to register so it is recommended you do so.