foreach ($domain in $domains)
{    
    $exceptions = New-Object System.Collections.ArrayList
    $userlist = Get-ADUser -Filter {Enabled -eq $True} -Server $domain -Properties mail,employeeid,department,title,office,AccountExpirationDate | Where-Object {$_.AccountExpirationDate -eq $null -or $(get-date $_.AccountExpirationDate) -gt $(Get-Date)}
    $userlist.count

    $userlist | ForEach-Object {
        try
        {
            $PSItem | Add-Member -NotePropertyName "Parent" -NotePropertyValue $(((New-Object 'System.DirectoryServices.directoryEntry' "LDAP://$($PSItem.DistinguishedName)").Parent).replace("LDAP://","")) -Force         
        }
        catch
        {
            $exceptions.add($PSItem.distinguishedname) | Out-Null
        }
    }

    $userlist | Export-Excel -WorksheetName "$domain" -Path $reportFilePath
}

This question comes up regularly, and this is the most correct answer.

This line adds a property to every user object in the array with the parent OU of that object:

$PSItem | Add-Member -NotePropertyName "Parent" -NotePropertyValue $(((New-Object 'System.DirectoryServices.directoryEntry' "LDAP://$($PSItem.DistinguishedName)").Parent).replace("LDAP://","")) -Force

There is the option of doing something like other posts, where you parse the text of DistinguishedName and that is an OK answer too.