Federated Sharing offers two ways to share information with recipients in external organizations (Outlook Live/Exchange Online): 1) Organization relationships and 2) Sharing policies.
You can select between the following levels of availability information shared using an organization relationship:
What’s great about this is you can scope which users to communicate with. It could be org-wide or you can specify a specific group like faculty. Availability information for your users who are member of the selected distribution group will be visible to all users in the external organization. Similarly, the external organization can create an organization relationship with your organization, and make the availability information of selected or all users available to users in your organization.
Once the relationship is established the autodiscover service is used to find the published availability service for the external web service. You can also configure this manually.
Again you can create the relationship either with EMC or the shell. The shell will use the New-OrganizationRelationship cmdlet.
Sharing policies are created by administrators to allow users to share calendar and contact information that resides in the respective folders with users in external federated organizations. Sharing policies contain pairs of domain names and the sharing actions that are allowed for users from that domain. You can specify the following actions that apply to the external domain specified in a sharing policy.
When creating a sharing invite for recipients from the external domain, your users can select the level of details they want to share, provided the action is allowed by the sharing policy that's applicable to the user.
Sharing policies can be disabled or enabled. When a sharing policy is disabled, sharing is stopped for all users who have that policy applied.
Once this is complete we have federated sharing for free/busy. More information can be found in Technet: http://technet.microsoft.com/en-us/library/dd351109(EXCHG.140).aspx