Starting today, new Google+ communities will be restricted to your domain by default if sharing is restricted in the Admin console. Posts in these communities are only viewable by people within the domain. However, you can still choose to create communities with people outside your organization so clients, agencies or business partners can join in. You can also make your community open to anyone at your domain or private, joinable by invitation only. Community owners can easily change settings, manage membership or invite other team members to join.

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

For more information:
http://googleenterprise.blogspot.com/2013/11/private-conversations-with-restricted.html
https://support.google.com/plus/answer/3379754
https://support.google.com/a/answer/2677328

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