Sharing settings control the sharing of Exchange information (specifically, calendar information such as free/busy, and more detailed information about appointments) with people in other domains:
Sharing settings
You can create a relationship between your organization and another one that will apply to either all of your users, or a specific security group:
New organization relationship
You can enable the sharing of free/busy information with the time only, or with the subject and location, as well. Limit sharing to a specific security ...
