It's about time Google+ (or Google Plus) now available using Google Apps domain. This includes Google Apps (Standard), Google Apps for Business, Google Apps for Government, and Google Apps for Education.
The Google Apps for domain administrator needs to turn-ON this feature. Here's how to.
- Log in to the Google Apps administrator control panel.
- Click the Organization & users tab.
- Click the Services tab.
- Scroll down 'till you see the Google+ icon, change it to ON.
- You're done.
Source: (2011, Whaddya.com)