Enter the Users area by clicking the "Users" button in the spine to the left.
Hove your cursor over the "Users" folder and click the three black dots to the right.
In the panel that expands out, click create.
The will redirect you to a main screen where you can enter the users login name, email, and then click save. I generally use the users NID for their login name.
(optional step) Afterwards, I like to expand out the users directory and search for the user we just created. Leave his login name the same, but be sure to change his actual name to not be his NID as this will make it easier to search for.