I was trying to turn off my Macbook Pro and suddenly got this message. As far as I know, there's no reason why this should be happening. Anyone want to fill me in on some possibilities?
Apparently, you've enabled File Sharing in the Sharing section of System Preferences. Someone on your network has connected to your computer, either as a registered user or as a guest.
Unless this kind of access is absolutely required, it's good security practice to disable both File Sharing and the Guest user in System Preferences -> Sharing and System Preferences -> Accounts and Groups, respectively.