This accepted answer is outdated. It is now possible to share data between users of a given app in Windows 10, by changing a setting in Group Policy Editor. The path in GP Editor (gpedit.msc) is:
\ Local Computer Policy \ Administrative Templates \ Windows Components \ App Package Deployment \ "Allow a Windows app to share application data between users".
Set to "Enabled", and an app can share its data across users.
Once enabled, in UWP apps, the path given by Windows.Storage.ApplicationData.Current.SharedLocalFolder.Path is available for sharing data across users. When not enabled, the path returned by this is Null.