Private courses, tests and live classes are not visible in the public marketplace and as a student, you cannot enroll without an invitation. When you receive an invitation by e-mail from the instructor, make sure you login or create an account with the same e-mail address where you have received the invitation to enroll in the private course, test or a live virtual classroom class. If you login with a different e-mail address, the "private" content will not be linked with your account and still remain in 'hidden' state.
Once you login with the same e-mail address where you have received the course invitation, you may still need to buy the "Paid" course to get full access. The same applies to all private contents shared with you by the instructor securely.