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Currently, uwp provides FindAllAsync method returns the user that is currently logged-in and using the app for most devices, it doesn't return all the accounts on the machine. There is DirectoryEntry class under System.DirectoryServices namespace could get all users of a machine, but the System.DirectoryServices is not supported on uwp. So maybe you need to create a win32 app uses the System.DirectoryServices and package it into uwp app, then you could use FullTrustProcessLauncher Class to call this win32 app.
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