Until today, Google Apps for Business and Education customers had access to Google Groups for Business, which's part of the core Google Apps suite and allowed the creation of groups and mailing lists within the organization. Now, Google introduced the general Google Groups service, to free version of Google Apps as well as Google Apps for Business and Education, which allows users to participate in public discussions with others outside the organization, available with Google Apps.
"Google Groups allows the creation of groups and mailing lists with any group of people from all across the web and includes Usenet archives and archives of public groups. For e.g., if you're writing a new mobile app and want to follow the Android Developers forum or are interested in the latest news about Chrome OS in the Chrome notebook pilot forum you can use Google Groups to get involved in the discussion. There're millions of public groups available, so no matter your field or focus, you're likely to find relevant discussions that you can follow and join using Google Groups. You can also set up your own public discussion community using Google Groups," explains Google.
Administrators can turn on both services, or only the one that makes most sense for their use case, giving more control over how users participate in groups inside and outside of the organization. Google Groups can be enabled by domain admin from the Google Apps Control Panel at https://www.google.com/a/[your_domain.com] (replace [your_domain.com] with your actual domain name).
More Info: Help Center
[Source: Google Enterprise blog]