Guests in your academy have very limited capabilities in comparison with the academy members. Specifically, they can only view the courses that are public to guests and all of them view the same set of training since they do not have jobs, org nodes’ associations, etc.
The main use of that kind of account is for evaluating people before you hire them. So, you can, for example, add those users in the platform as guests, have them take a test, and if they pass with a certain score, hire them, and convert them to regular members within the academy. So, the goal is usually to have those users initially into the academy temporarily as guests and then, convert them to regular members.
Guests have access only to Courses (not Curricula) and can see lectures, exams, and polls but not on-the-job trainings or events.
They can only self-enroll in courses that are available for them, meaning that you, as an admin, do not have the ability to assign courses to external members.
Please note that the courses that are public for guests need to also belong to the academy course category "Without Category" in order to be visible to them. You can learn more on how to make courses public for guests here: Course Visibility