If your course is in Private mode only people who either get an invitation or are assigned the course can see it. Nobody else. Even if those invited share the invitation link with others they won't be able to see your course.
You can send invitations from the Course Administration Panel either to friends you have on UltiPro Learning or to people outside of UltiPro Learning. You can send invitations for free or paid courses. For paid courses, you can also send coupons that are invitations for special discounts.
If your course is in Public mode (e.g. you have published it), then everybody on UltiPro Learning can see it. For corporate courses, however, you can choose to publish the course selectively only to a certain group of people (e.g. employees, customers). In the latter case, only those groups of people will be able to see your course on the appropriate pages. For example, if you create a corporate course and publish it only to employees, the course won't show up in the UltiPro Learning Public course library; it will show up only on the Academy's course page, and only employees will be able to see it.
If you published a corporate course to customers, then customers will be able to see this course only on the Academy's course page. Even though they may have access to the Academy's course page, they won't be able to see corporate courses that are visible only to employees.