Accounts and users are allocated to Sites, and are assigned to Security Groups.
On the top right of the screen, click on the arrow to bring up a list of Sites allocated to your Account
Click on the Site name where you want to add the user. This will change the current site on the console
When the Site is selected, click on "Manage Site Users" to bring up a list of users on that site
To add or change permissions
The Site Users screen shows the users allocated to the Site, their current permissions in green, and other permissions not assigned in red
Add new permissions by clicking on the + icon, and remove existing permissions by clicking on the - icon