Building a corporate intranet - group permissions and authoring

I will apologize ahead of time if I am asking something that should have been easy to find. I've searched the Drupal forums and the web to no avail. While I've read in several places that Drupal provides facilities for group permissions, I've only found references to roles and the ability to assign them to blocks. I am looking to build a corporate intranet with portions of the web site for each department. Each department would identify the one or two people who should have editing permissions on their portion of the web site. What module or modules do I need to restrict editing permissions on content to specific groups of users?

Many thanks.
Scott