Microsoft announced the release of Microsoft Whiteboard – Preview version for Windows 10.  This app allows multiple users to sign into their Microsoft accounts and remotely update a virtual whiteboard to collaborate visually.  This app is free to use for any one user with a Windows 10 device.  For multiple users, at least one participant must be logged into an Office 365 personal, work, or school account.  More details are at products.office.com/en-us/microsoft-whiteboard/digital-whiteboard-app and the FAQ page.

Update: Microsoft Whiteboard app is now generally available for Windows 10 devices at the Microsoft Store, and there is now an iOS version for iPhone and iPad.