Admins now have the ability to customize which Hangouts features are available to which employees. They choose to limit Hangouts chat messages to being internal-only, set chat history to off by default and decide whether users within the domain can contact each other without sending or accepting formal invitations first. Video and audio chat can also be turned off across the organization.

Hangouts also supports auto-complete through the global address list. You can start by typing the name of anybody in your organization and auto-complete will help you find who you're looking for.

Editions included:
Google Apps for Business, Education, and Government

For more information:
https://support.google.com/a/answer/3094760
http://googleenterprise.blogspot.com/2013/11/making-it-easier-to-bring-hangouts-to.html