Creating a custom role for user

I have a fairly good understanding of the standard user roles offered – Admin, Editor, Author, Contributor and Subscriber.

What I want to have is the ability to assign a user to a specific section of a website. For an example, the website i am referring to is http://www.columbusmuseum.org. I want to assign the role of an Editor to – http://www.columbusmuseum.org/center-for-creativity/
only. I just want the editor to log-in and have only the ability to edit this page and the children of that page.

Please advise.
Scott