Ruby help required to realize login and sign-up options for user

I am using ruby on rails and I would like to know if it is possible to implement a form where user can register if it is first time visit, and login if he has already registered. The user will enter username and password and send a POST. It will be verified in the back ground and will allow the user to login if it is validated successfully. Currently i am facing error in authgem. I am trying to use JSON request and response to achieve my objective. So, this is basically a login page for the user to access my website. Is there any other solutions available? Thanks