In a new Office Blog post Microsoft elaborates on it's plans to introduce Cortana to Office 365. They believe her ability to learn your preferences, to provide relevant recommendations, fast access to information and important notifications makes technology truly personal and work on behalf of the customer.
With that in mind, Cortana in Windows 10 will currently connect to Office 365 for businesses to "help you accomplish more work". You can preview this today if your business is opted in to the First Release for Office 365 and you are a Windows Insider running Windows 10.
Cortana can already help you prepare for meetings. Now with Office in her portfolio, she can let you know what documents the people you're meeting with have been working on. Presumably with their consent. Importantly, Office 365 admins will have the option to opt-out of Cortana connectivity in the Office 365 Admin portal.