Linux is a multi-user operating system by default. System users, human users, groups and a lot more.
As a sysadmin, you should know how to get details of users on the system, modify their access, change their passwords or set a password policy and manage their groups.
You'll learn about various Linux commands that allow you to manage user accounts and groups in this section.
I recommend reading a little on the file ownership and groups. This will give you a better understanding on groups at least.