Adding users and changing permissions is done in the Users section on the sidebar. ON this page there is a list of all users which can be sorted by user groups
You can select any user and suspend, unsuspend, or delete them by clicking the checkbox next to the user and then the gear icon at the top of the page. Suspended users lose all permissions until unsuspended. Deleted users are deleted permanently so be careful about choosing this option.
Create a new user by clicking the red New user button at the top right of the page.
Fill in their username and email address, and optionally first/last name. Then switch to the permissions tab. Here you can select the pre-made user group of Site Admin or any other groups that may be created in the future. Or you can manually check off each permissions option you want for the new user.
When you save the user will receive an activation email where they can create their password. for users that have not yet activated their account you additional options by opening their profile and then clicking the gear icon on the right hand side where you can:
Most times these options will not be necessary.