PS Ver. 5 und Method invocation failed because System.Management.Automation PSCustomObject does not contain a method named Where

#Get Group membership
$Header |
ForEach-Object{
   $Group = $_
   $addADGroupMembers.$Group = $xlsxADGroupMembers.Where{$_.$Group -eq '1'} | Select-Object -ExpandProperty 'UserID'  
   $delADGroupMembers.$Group = $xlsxADGroupMembers.Where{$_.$Group -ne '1'} | Select-Object -ExpandProperty 'UserID'  
}

Method invocation failed because [System.Management.Automation.PSCustomObject] does not contain a method named 'Where'.  
At C:\AddGroupMemberExcel.ps1:38 char:4
+    $addADGroupMembers.$Group = ($xlsxADGroupMembers).Where{$_.$Group  ...
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (Where:String) , RuntimeException
    + FullyQualifiedErrorId : MethodNotFound

<#

Author: I. Strachan
Version:
Version History:

Purpose: Add group member from excel worksheet. Scripts is based on Mike Robin's work  
         http://tinyurl.com/jfmw4o7 
#>
[CmdletBinding()]
param(
   $xlsxFile = 'Test.xlsx',  
   
   $WorkSheet = 'Test'  
)

#region
$exportDate = Get-Date -Format ddMMyyyy
#endregion

#Import Worksheet Using D. Finke's ImportExcel module http://tinyurl.com/lbhkhbd  
$xlsxADGroupMembers = Import-Excel "C:\$xlsxFile" -WorkSheetname $WorkSheet  

#Select Group names from Object
$Header = $xlsxADGroupMembers |
   Get-Member -MemberType NoteProperty | 
   Where-Object{$_.Name -ne  'UserID'} |   
   Select-Object -ExpandProperty Name

#Create empty hashtables
$addADGroupMembers = @{}
$delADGroupMembers = @{}

#Get Group membership
$Header |
ForEach-Object{
   $Group = $_
   $addADGroupMembers.$Group = $xlsxADGroupMembers.Where{$_.$Group -eq '1'} | Select-Object -ExpandProperty 'UserID'  
   $delADGroupMembers.$Group = $xlsxADGroupMembers.Where{$_.$Group -ne '1'} | Select-Object -ExpandProperty 'UserID'  
}

#region Main. Add and remove users to/from groups
$Header |
ForEach-Object{
   if($addADGroupMembers.$_){
      try{
         Add-ADGroupMember -Identity $_ -Members $addADGroupMembers.$_
      }
      catch [Microsoft.ActiveDirectory.Management.ADIdentityNotFoundException]{
         Write-Warning "AD Object $($Error.CategoryInfo.TargetName) not found"  
      }
   }

   if($delADGroupMembers.$_){
      try{
         Remove-ADGroupMember -Identity $_ -Members $delADGroupMembers.$_ -Confirm:$false
      }
      catch [Microsoft.ActiveDirectory.Management.ADIdentityNotFoundException]{
         Write-Warning "AD Object $($Error.CategoryInfo.TargetName) not found"  
      }
   }
}
#endregion

#region Export for futher processing
$GroupMembers =@{
  Groups  = $Header | Get-ADGroup -Properties WhenCreated
  Added   = $addADGroupMembers
  Revoked = $delADGroupMembers
}

$GroupMembers | 
Export-Clixml "C:\ADGroupMembers-$exportDate.xml" -Encoding UTF8  
#endregion