If you've activated Google+ premium features in your organization, you can choose whether hangouts are restricted to your organization by default. If this restriction is set, users will see a notice when starting a hangout that it's restricted. However, note that users can still override this and either invite only specific external participants, or open the hangout to the public.

• If you don't restrict hangouts to your organization by default, anyone who has the URL to the hangout can freely join, including people outside your organization, as long as the hangout is not full.
   
• If you do restrict hangouts by default, people within your organization can always join with just the URL, but people outside your organization must be invited.

To restrict hangouts in your organization by default:

1. Sign in to the Google Admin console.
2. Do one of the following:
   • In the classic Admin console, click Settings > Google+.
   • In the new Admin console, click Other Google services > Google+.
     ^{Where is it? Which Admin console do I have?}

3. On the Org Settings tab, select the organization for which you want to set the Hangouts option.

   Select the top-level organization to enforce the same settings for your entire organization. You can also specify different settings for each organizational unit. If you have a lot of organizations, click the + button to expand your options.

   Under Settings in the left column, locally applied means that the settings are not inherited from the parent organizational unit. (Settings at the top-level organization are always locally applied.) Inherited means that the settings are taken from the parent.

4. In the Hangout options section, select People outside of my organization can't join a hangout unless invited.