EASY WAY TO ADD THE ADMINISTRATOR USER TO THE WELCOME SCREEN.!!

Start the Registry Editor Go to:
HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows NT \ CurrentVersion \ Winlogon \ SpecialAccounts \ UserList \
Right-click an empty space in the right pane and select New > DWORD Value Name the new value Administrator. Double-click this new value, and enter 1 as it's Value data. Close the registry editor and restart.