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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.
See all the folders they are sharing (Computer Management -> System Tools -> Shared Folders -> Shares); See who (from their network) is connected to their computer (Computer Management -> System Tools -> Shared Folders -> Sessions); What shared files are opened (Computer Management -> System Tools -> Shared Folders -> Open Files).