Site > User Management and click the new icon in the top right hand corner of the screen
The user creation screen should appear with the options to create a new user.
Fill in the fields with the appropriate details and hit save in the top right corner of the screen.
The group of the user will depend on the level of access the user requires.
As a default, all front end users should be set to Registered. All backend administrators should be set to Administrator. Site Administrators should be set to Super Administrator.
Also make sure that the user is NOT blocked and that they are not receiving system emails unless they are a super administrator.