Hi Adam
There are two ways this could be achieved
Hiding the course under course visibility and changing the learner role for the course to see hidden courses
Enabling audience based visibility (Site admin > Configure features > Shared services settings and then setting the course visibility setting to Enrolled users only
The audience based visibility option is a bit more flexible as this can also be used for programs and certification. restricting visibility to Audiences members is also able to be included,
regards