You may have the option to make your courses public or private. For users of the free version, courses will always be public. Paid users will have the option to make courses either public or private.
You my do this from the Course Dashboard.
You may also set course visibility from the "Settings" menu inside the course.
A public course may be taken by anyone with the course url. There is no limit to the amount of people that can take a public course.
A private course may only be seen and taken by users specifically invited via email. Please note that inviting and tracking learners is a paid feature. The free version does not provide those services.