I need to join a classroom Japanese learning
Classrooms are set-up by teachers in schools and are meant for their students. If your teacher has not set-up a Duolingo classroom for you to use then I would be suspicious of joining someone else's classroom.
You would only join one if you have a class in the real world:
Classrooms offer nothing more than you can already access on your own.
A teacher of a classroom can access and control your account. They can limit what you have access to, change your password and see your e-mail address.