[Get Logged On User] Use WMI to find out, which user is logged in on a specific server.

Get-LoggedOnUser.ps1

foreach ($server in $serverNames)
{
    Get-WMIObject Win32_Process -Filter 'name="explorer.exe"' -ComputerName $server -PipelineVariable pipedProcess -ErrorAction SilentlyContinue | 
        Select CSName, @{ Name='User'; Expression={$pipedProcess.GetOwner().User} } | 
        Write-Output -OutVariable +loggedOnUsers
}