PowerShell try catch

I need you help Im using the next script: How to get all groups within all webs in SharePoint 2010 using Powershell

The script perfectly, but some user can not be found, so I'm trying to control the errors and send them to a file, I try:

catch [System.Exception]{
    "ERROR: " + $($_.Exception.Message)| out-file $logfile -append
}

But this does not work :( I appreciate your help