Synchronize/Mirror AD Security Group Membership Script

Copy-ADGroupMember.ps1; -WhatIf is supported.
Will by default only copy; use -Mirror to remove members from destination that aren't in the source anymore.
Use -Confirm:$False to suppress the confirmation prompt (or change ConfirmImpact to 'None' if you never want to be asked)

[CmdletBinding(SupportsShouldProcess=$True, ConfirmImpact='High')]
Param(
	[PSObject]$Identity,
	[PSObject]$Destination,
	[Switch]$Mirror
)
$SourceMembers = Get-ADGroupMember -Identity $Identity | Select-Object -ExpandProperty distinguishedName
$TargetMembers = Get-ADGroupMember -Identity $Destination | Select-Object -ExpandProperty distinguishedName
$DifferenceMembers = Compare-Object -ReferenceObject $SourceMembers -DifferenceObject $TargetMembers
$ProcessMembers = $DifferenceMembers | Where-Object {$_.SideIndicator -eq '<='} | Select-Object -ExpandProperty InputObject
If ($PSCmdlet.ShouldProcess($Destination, "Add objects:`r`n`t$($ProcessMembers -join "`r`n`t")`r`n")) {
	Add-ADGroupMember -Identity $Destination -Members $ProcessMembers
}
If ($Mirror) {
	$ProcessMembers = $DifferenceMembers | Where-Object {$_.SideIndicator -eq '=>'} | Select-Object -ExpandProperty InputObject
	If ($PSCmdlet.ShouldProcess($Destination, "Remove objects:`r`n`t$($ProcessMembers -join "`r`n`t")`r`n")) {
		Remove-ADGroupMember -Identity $Destination -Members $ProcessMembers
	}
}

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Thank you, where would I specify the source and destination group? Am I just replacing [PSObject] in lines 3 and 4?

No. It's a complete script accepting arguments.
.\Copy-ADGroupMember.ps1 -Identity SourceADGroup -Destination DestADGroup -Mirror -WhatIf

If you really want to hard code that, make a function out of it and call the function:

Function Copy-ADGroupMember {
[CmdletBinding(SupportsShouldProcess=$True, ConfirmImpact='High')]
Param(
	[Parameter(ValueFromPipeline=$True, Position=0)]
	[PSObject]$Identity,
	[PSObject]$Destination,
	[Switch]$Mirror
)
	$SourceMembers = Get-ADGroupMember -Identity $Identity | Select-Object -ExpandProperty distinguishedName
	$TargetMembers = Get-ADGroupMember -Identity $Destination | Select-Object -ExpandProperty distinguishedName
	$DifferenceMembers = Compare-Object -ReferenceObject $SourceMembers -DifferenceObject $TargetMembers
	$ProcessMembers = $DifferenceMembers | Where-Object {$_.SideIndicator -eq '<='} | Select-Object -ExpandProperty InputObject
	If ($PSCmdlet.ShouldProcess($Destination, "Add objects:`r`n`t$($ProcessMembers -join "`r`n`t")`r`n")) {
		Add-ADGroupMember -Identity $Destination -Members $ProcessMembers
	}
	If ($Mirror) {
		$ProcessMembers = $DifferenceMembers | Where-Object {$_.SideIndicator -eq '=>'} | Select-Object -ExpandProperty InputObject
		If ($PSCmdlet.ShouldProcess($Destination, "Remove objects:`r`n`t$($ProcessMembers -join "`r`n`t")`r`n")) {
			Remove-ADGroupMember -Identity $Destination -Members $ProcessMembers
		}
	}
}

.\Copy-ADGroupMember.ps1 -Identity SourceADGroup -Destination DestADGroup -Mirror -WhatIf

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Thank you. The script seems to work, but throws errors.  Can these be suppressed?

PS C:\scripts> .\Copy-ADGroupMember.ps1 -Identity SourceADGroup -Destination DestDL -Mirror -Confirm:$False
Remove-ADGroupMember : Cannot validate argument on parameter 'Members'. The argument is null or empty. Provide an
argument that is not null or empty, and then try the command again.
At C:\scripts\Copy-ADGroupMember.ps1:17 char:56
+ ...  Remove-ADGroupMember -Identity $Destination -Members $ProcessMembers
+                                                           ~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Remove-ADGroupMember], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationError,Microsoft.ActiveDirectory.Management.Commands.RemoveADG
   roupMember

PS C:\scripts> .\Copy-ADGroupMember.ps1 -Identity SourceADGroup -Destination DestDL -Mirror -Confirm:$False
Add-ADGroupMember : Cannot validate argument on parameter 'Members'. The argument is null or empty. Provide an
argument that is not null or empty, and then try the command again.
At C:\scripts\Copy-ADGroupMember.ps1:12 char:52
+     Add-ADGroupMember -Identity $Destination -Members $ProcessMembers
+                                                       ~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Add-ADGroupMember], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationError,Microsoft.ActiveDirectory.Management.Commands.AddADGrou
   pMember

PS C:\scripts>

[CmdletBinding(SupportsShouldProcess=$True, ConfirmImpact='High')]
Param(
	[PSObject]$Identity,
	[PSObject]$Destination,
	[Switch]$Mirror
)
$SourceMembers = Get-ADGroupMember -Identity $Identity | Select-Object -ExpandProperty distinguishedName
$TargetMembers = Get-ADGroupMember -Identity $Destination | Select-Object -ExpandProperty distinguishedName
$DifferenceMembers = Compare-Object -ReferenceObject $SourceMembers -DifferenceObject $TargetMembers
$ProcessMembers = $DifferenceMembers | Where-Object {$_.SideIndicator -eq '<='} | Select-Object -ExpandProperty InputObject
If ($ProcessMembers -and $PSCmdlet.ShouldProcess($Destination, "Add objects:`r`n`t$($ProcessMembers -join "`r`n`t")`r`n")) {
	Add-ADGroupMember -Identity $Destination -Members $ProcessMembers
}
If ($Mirror) {
	$ProcessMembers = $DifferenceMembers | Where-Object {$_.SideIndicator -eq '=>'} | Select-Object -ExpandProperty InputObject
	If ($ProcessMembers -and $PSCmdlet.ShouldProcess($Destination, "Remove objects:`r`n`t$($ProcessMembers -join "`r`n`t")`r`n")) {
		Remove-ADGroupMember -Identity $Destination -Members $ProcessMembers
	}
}

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Thanks, that resolved those errors.  However I just tested the script, and find that when the destination group has no members yet, it fails to populate with members in the source group.  It works when the destination group contains at least one member.  here is the error I get:

PS C:\scripts> .\Copy-ADGroupMember.ps1 -Identity SourceADGroup -Destination DestDL -Mirror -Confirm:$False
Compare-Object : Cannot bind argument to parameter 'DifferenceObject' because it is null.
At C:\scripts\Copy-ADGroupMember.ps1:13 char:87
+ ... ject -ReferenceObject $SourceMembers -DifferenceObject $TargetMembers
+                                                            ~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Compare-Object], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorNullNotAllowed,Microsoft.PowerShell.Commands.CompareObje
   ctCommand

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Please note that the line 13 referenced in the error is

$DifferenceMembers = Compare-Object -ReferenceObject $SourceMembers -DifferenceObject $TargetMembers

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(I added 4 commented lines to the top of your script when I ran, so for your original script line error would likely be line 9) Hope this didn't confuse you!

Works beautifully, thank you!