Office 365: Windows Azure Active Directory

When you subscribe to an Office 365 Plan Microsoft creates a new tenant for you. Microsoft also automatically creates a new Windows Azure Active Directory (WAAD) that is associated with your Office 365 tenant. According to Microsoft Windows Azure Active Directory (Windows Azure AD) is a modern, REST-based service that provides identity management and access control capabilities for your cloud applications. Windows Azure Active Directory provides a cloud-based identity provider that easily integrates with your on-premises AD deployments and full support of third party identity providers.

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Well, when you subscribe to a Yammer Plan with Office 365 you are not on the same tenant and you are NOT using WAAD. Yammer has another Idendity Store.