Note that this code MUST run on a SP server front end.

                using Microsoft.SharePoint.Administration;              

                //
                // Check if we are on a SP server farm or not to get default server URL
                //
                SPFarm farm = SPFarm.Local; 

                 if (farm != null)
                {   

                    // Get the collection of all SP Alternate Urls        
                    SPAlternateUrlCollectionManager altUrlCollectionMgr = farm.AlternateUrlCollections;

                    // iterate thru collections of Urls
                    foreach (SPAlternateUrlCollection altColl in altUrlCollectionMgr)
                    {
                        foreach (SPAlternateUrl url in altColl)
                        {
                            // check if url is in Default zone
                            if (url.UrlZone == SPUrlZone.Default)
                            {
                                // get the port of the Uri and check for port 80
                                int UrlPortValue = url.Uri.Port;
                                if (UrlPortValue == 80)
                                {
                                    // set serverURL to Uri with port of 80
                                    this.serverURL = url.Uri.AbsoluteUri;
                                    // 
                                    return serverURL;
                                }
                            }
                        }
                    }

In the below example I contact the AD Global Catalog (GC) for the current domain and search for the AD "mail" attribute. The GC contains limited records of every AD object in the Forest. If I find the object I then get it's Distinguished Name (DN). The DN that returns can be from a domain other than the current domain (ex. cn=foo,dc=HQ,dc=contoso,dc=com). Then I cast the DN to an ADSI object so I can get/update it's properties. This gets you around the referral error!

# Contact AD Global Catolog to search for email address
 $dom = [System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()
 $root = [ADSI]"GC://$($dom.Name)"
 $search = new-Object System.DirectoryServices.DirectorySearcher($root,"(mail=$stmail)")
 $result = $search.FindOne()

 if($result -ne $null)
 {
  $user = $result.GetDirectoryEntry()
  #write-host "User: " $user.distinguishedName " found" -ForeGroundColor Green;
 
  # cast each member as an ADSI object
  $user2 = $user.distinguishedName
  $adsPath = 'LDAP://' + $user2
  $account=[ADSI]$adspath
  $account.setinfo()
   
  #update all users
  $account.put($ManagerURI, $GrooveManager)
  $account.setinfo()
  write-host " Provisioned account: " $account.distinguishedName " " -ForeGroundColor Green; 
  $counter++

 }
 else
 {
  #$stmail
  write-host " Error: Member Email does not exist in Active Directory Global Catalog" -ForeGroundColor Red;
 }

 # Create XML document from file
$MemberDoc = [xml](get-content $XMLFile)

# Get all of the email addresses in the XML document
$EmailAddresses = $MemberDoc.GetElementsByTagName("Email")

Foreach ($Email in $EmailAddresses){

 # Get InnerXML value of Email node and assign it to string
 $stmail=$Email.InnerXML

}

# $LDAPGroup is in the format cn=TestGroup,dc=contoso,dc=com 
$container = 'LDAP://' + $LDAPGroup
$member = 'member'
$counter=0

$GroupEntry = New-Object DirectoryServices.DirectoryEntry($container)
$GroupSearcher = New-Object DirectoryServices.DirectorySearcher($GroupEntry)
$GroupSearcher.Filter="(objectClass=*)"

$rangeStep = 1000
$rangeLow = 0
$rangeHigh = $rangeLow + ($rangeStep - 1)
$lastQuery = $FALSE
$quitLoop = $FALSE

do
{
 if(!$lastQuery)
 {
  $attribRange = 'member;range=' + $rangeLow + '-' + $rangeHigh
 }
 else
 {
  $attribRange = 'member;range=' + $rangeLow + '-*'
 }

 # clear searcher properties and add new range
 # cast to [void] so result won't display

 [void]$GroupSearcher.PropertiesToLoad.Clear()
 [void]$GroupSearcher.PropertiesToLoad.Add($attribRange)

 $Results = $GroupSearcher.FindOne()

 If ($Results -eq $Null)
 {
  write-host " "
  write-host "Error: Unable to find specified LDAP Group" -ForeGroundColor Red;
  write-host " "
  break
 }
 
 foreach($Res in $Results.Properties.PropertyNames)
 {
  # for debugging range
  #write-host "member range: " $Res
 }

 if($Results.Properties.Contains($attribRange))
 {  
  foreach($Res2 in $Results.Properties[$attribRange])
  {
   # cast each member as an ADSI object
   $adsPath = 'LDAP://' + $Res2
   $account=[ADSI]$adspath
   $account.setinfo()
 
   # use PSObject to display the properties of the object

   $obj = New-Object PSObject
 
   $obj | Add-Member -MemberType NoteProperty -Name sAMAccountname -Value $account.properties.samaccountname[0]
   $obj | Add-Member -MemberType NoteProperty -Name DN -Value $account.path
   write $obj
   $counter = $counter + 1

  }
  if($lastQuery)
  {
   $quitLoop = $TRUE
  }
 
 }
 else
 {
  $lastQuery = $TRUE
 }

 if(!$lastQuery)
 {
  $rangeLow = $rangeHigh + 1
  $rangeHigh = $rangeLow + ($rangeStep - 1)
 }

}
until($quitLoop)

write-host " "
write-host "Total users:" $counter