You can use Active Directory Federation Services (AD FS) to provide users on your network with access to federated applications ...

You can use Active Directory Federation Services (AD FS) to provide users on your network with access to federated applications that are located in a partner organization by adding the necessary entries to the Federation Service in your organization. After you add the entries to the Federation Service and establish a federation trust relationship between both organizations, internal users will be able to access multiple federated applications in the partner organization using a single sign-on.