Official Google Enterprise Blog: Google+ is now available with Google Apps

Google+ for organizations

Starting now you can manually turn on Google+ for your organization. Once Google+ is turned on, your users will just need to sign up at google.com/+ to get started. For customers who use Google Apps for Business or the free version of Google Apps and who have chosen to automatically enable new services, Google+ will automatically become available to all of your users over the next several days.*
Google Apps users will have access to the same set of features that are available to every Google+ user, and more. In addition to sharing publicly or with your circles, you’ll also have the option to share with everyone in your organization, even if you haven’t added all of those people to a circle.

Official Google Enterprise Blog: Google+ is now available with Google Apps