The tools are available for free through the end of 2013, but users will then need a paid subscription to Google's cloud-based software for businesses, Google Apps, for continued access.
Here's a rundown of the new features:
Private sharing. When you create a post, you can now label it as "restricted." This will limit its visibility to those inside your organization (plus a few external partners if you so choose), and can't be re-shared with anyone on the outside.
Administrative controls. Administrators can set up company-wide sharing defaults for posts and Hangouts.
Hangout & Calendar integration. Since July, Google+ users have been able to organize Hangouts directly from Gmail, and open Google Docs documents during those meetings. Now, users can attach Hangouts to Google Calendar events, enabling attendees to join a Hangout directly from the Calendar entry or original invite (see below).
In a blog post, Google said these new features were just the beginning. In the coming months, the company is planning to introduce a mobile version of Google+ for enterprise, as well as additional administrative controls.