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Here is the official word from Microsoft TechNet, The C:\Users folder is shared when any sub folders from C:\Users\<username>\ is shared. The article is stale and mostly unhelpful, but worth knowing to complete the story.
Windows Vista. Set shared notebook permissions on Windows Vista. Each user with whom you want to share your notebook must have a user account and password on the same network as you. In Windows Vista, click the Start button, and then click Documents.In the Documents folder, open the OneNote Notebooks folder.ote If you have never before shared a folder on your network, the Share this folder on the network check box may not appear until you acknowledge a security warning and select the option Just enable file sharing. In the Share Name box, verify the name of the notebook. It is not necessary to change the default name.