Found Here: https://www.vspbreda.nl/nl/server-os/server-2012-essentials-users-dont-show-in-essentials-dashboard-after-migration-solved/ 

Issue. You add a User to AD but it does not show up in the Logon list in the Dashboard

Solution: Run PowerShell as Admin and run this command. Import-WssUser –SamAccountName “username”