A note on users and roles to answer your question:
Anyone with a username and password is an authenticated user in Moodle. As such, they can see some bits of the Moodle site, but not all. Access to a category or course is managed through assigning roles to a user for these contexts.
A user on your Moodle needs to be authenticated, or else he/she would be anonymous and no data whatsoever can be recorded to gain insight into a user's progress in a course.
Any user with a role as manager, teacher or student is considered a participant in a course.
There's probably a bit on this in the videos for beginner Moodle admins Mary shared in one of your other questions.