A good way to retrieve users of remote machines could be to filter the process for explorer.exe:

Get-WmiObject -class win32_process -Filter "name = 'Explorer.exe'" -ComputerName MACHINENAME -EA "Stop" | % {$_.GetOwner().User}

For all logged on users though, use the following script: https://gallery.technet.microsoft.com/scriptcenter/d46b1f3b-36a4-4a56-951b-e37815a2df0c

function Get-LoggedOnUser {
#Requires -Version 2.0          
[CmdletBinding()]          
Param            
   (                      
    [Parameter(Mandatory=$true,
               Position=0,                        
               ValueFromPipeline=$true,          
               ValueFromPipelineByPropertyName=$true)]          
    [String[]]$ComputerName
   )#End Param

Begin          
{          
Write-Host "`n Checking Users . . . "
$i = 0          
}#Begin        
Process          
{
    $ComputerName | Foreach-object {
    $Computer = $_
    try
        {
            $processinfo = @(Get-WmiObject -class win32_process -ComputerName $Computer -EA "Stop")
                if ($processinfo)
                {  
                    $processinfo | Foreach-Object {$_.GetOwner().User} |
                    Where-Object {$_ -ne "NETWORK SERVICE" -and $_ -ne "LOCAL SERVICE" -and $_ -ne "SYSTEM"} |
                    Sort-Object -Unique |
                    ForEach-Object { New-Object psobject -Property @{Computer=$Computer;LoggedOn=$_} } |
                    Select-Object Computer,LoggedOn
                }#If
        }
    catch
        {
            "Cannot find any processes running on $computer" | Out-Host
        }
     }#Forech-object(ComputerName)      

}#Process
End
{

}#End

}#Get-LoggedOnUser