Self-authorising employees are users who can submit holiday requests that will automatically be approved upon submission.
This also means that self-authorising users wouldn't be "member's" of a holiday group.
You can find out more information about Holiday Groups here.
Setting an Employee as Self-Authorising
You can set an employee as self-authorising by navigate to
Setup & Admin > Self-Authorising Employees.
In here, tick the person you'd like to set as self-authorising and press 'Submit'.
This will then set the ticked users as self-authorising.