Workbench Access provides the ability to segment editing access. These editorial sections of the site can be configured by role and/or specific user.
First, enable Workbench and Workbench Access modules.
The first decision that has to be made is what editorial sections (or "buckets") you want to assign content to. The choices are to use the "Main menu" for this (which can be unwieldy if you have a big menu) or create a special Taxonomy made up of your section labels. This choice is configured at Configuration->Workbench Access->Settings (admin/config/workbench/access/settings).
The Editors and Roles tabs are then used to assign specific users or entire roles, respectively, to editorial sections. For starters you'll want to give Site Admin permission to edit all sections: under the Roles tab (admin/config/workbench/access/roles) click the "0 roles" link to the right of the first section, then check Site Admin and save.
Much more can be found in the official docs.
Each of your pages will now need its editorial section set on its edit page. When you edit any page you'll have a field for this new editorial sectioning capability that allows you to assign the page to your choice of sections limited by the sections you have access to. Upon enabling of Workbench Access this field will be empty and will then need to be set before the page is saved. The limitation is that currently there is not a "mass assignment" capability. Therefore if you are establishing a section of your site that a certain editor will be limited to, a Site Admin will have to edit each page in that section and set its editorial section before the editor can take over.