I am on a network at an office I am visiting. I opened my Aperture app and plugged in my camera to import some photos. When I hit import the tab opened showing my camera and another computer on the same network. Is someone else accessing my Aperture and how can I prevent that?
You can see them because they have file sharing or some other service turned on. Unless you have turned on some sharing options (like file sharing, remote login, Remote Management, etc…) they can't see your computer on the network. If you have turned on some sharing options then they can see your computer, but probably can't access anything outside of your "/Users//Public" folder on it.