How to assign permissions for staff users
Hover over the account in question, then click the "Edit User" button and choose "Edit Permissions".
You'll then see a list of all available permissions that can be enabled and disabled.
Here, you can dig in and tweak them to your heart's content. Simply start in the area that you're looking for, and drill down until you find the permissions setting that will meet your needs.
As seen here, you can toggle access to entire modules on and off:
Or you can drill down much deeper. For example, by clicking the "SEO" option, you can choose whether to toggle its sub-modules on or off for this user:
And you can drill even deeper than that. For example, by drilling down through SEO --> Rankings, you can choose whether to allow this staff user the ability to manage keywords:
When finished setting permissions, click the "Save" button to save and exit.