LocalGroupmembership.ps1

 function Set-LocalGroupMember {
[CmdletBinding(SupportsShouldProcess=$true,ConfirmImpact='High')]
param(
	[Parameter(ValueFromPipeline=$true, Position=0)]
	[string[]]$ComputerName = '.',
	[Parameter(ValueFromPipeline=$false, Position=1)]
	[string[]]$GroupName = 'Administrators',
	[Parameter(ValueFromPipeline=$false, Position=2)]
	[string]$Member,
	[Parameter(Mandatory=$true, ParameterSetName='remove')]
	[switch]$Remove,
	[Parameter(Mandatory=$true, ParameterSetName='add')]
	[switch]$Add
)
BEGIN {
	$Domain, $User = $Member -split '\\'
	if (-Not $User) {
		$User = $Domain
		$Domain = $env:USERDOMAIN
	}
	$Member = ('{0}/{1}' -f $Domain.ToUpper(), $User)
	$MemberADSI = [ADSI]"WinNT://$Member,user"
	if (-Not $MemberADSI.Path) {
		throw "Can't find $Member ($Domain/$User)"
	}
	Write-Verbose ('User: ' + $MemberADSI.Path) # test for existance
}
PROCESS {
	foreach ($computer in $ComputerName) {
		foreach ($group in $GroupName) {
			$grp =[ADSI]"WinNT://$computer/$group,group"
			Write-Verbose ('Group: ' + $grp.Path) # test for existance
			if ($Add) {
				if ($PSCmdlet.ShouldProcess($computer, "Add $Member to $group")) {
					Write-Verbose "Adding $Member to $group @ $computer"
					$grp.Add($MemberADSI.Path)
				}
			}
			else {
				if ($PSCmdlet.ShouldProcess($computer, "Remove $Member from $group")) {
					Write-Verbose "Removing $Member from $group @ $computer"
					$grp.Remove($MemberADSI.Path)
				}
			}
		}
	}
}
}

function Add-LocalGroupMember {
[CmdletBinding(SupportsShouldProcess=$true,ConfirmImpact='High')]
param(
	[Parameter(ValueFromPipeline=$true, Position=0)]
	[string[]]$ComputerName = '.',
	[Parameter(ValueFromPipeline=$false, Position=1)]
	[string[]]$GroupName,
	[Parameter(ValueFromPipeline=$false, Position=2)]
	[string]$Member
)
BEGIN {
	$PSBoundParameters.Add('Add', $true)
	$list = New-Object System.Collections.ArrayList
}
PROCESS {
	[void]$List.Add($ComputerName)
}
END {
	$PSBoundParameters['ComputerName'] = [array]$List
	Set-LocalGroupMember @PSBoundParameters
}
}

function Remove-LocalGroupMember {
[CmdletBinding(SupportsShouldProcess=$true,ConfirmImpact='High')]
param(
	[Parameter(ValueFromPipeline=$true, Position=0)]
	[string[]]$ComputerName = '.',
	[Parameter(ValueFromPipeline=$false, Position=1)]
	[string[]]$GroupName,
	[Parameter(ValueFromPipeline=$false, Position=2)]
	[string]$Member
)
BEGIN {
	$PSBoundParameters.Add('Remove', $true)
	$list = New-Object System.Collections.ArrayList
}
PROCESS {
	[void]$List.Add($ComputerName)
}
END {
	$PSBoundParameters['ComputerName'] = [array]$List
	Set-LocalGroupMember @PSBoundParameters
}
}

function Get-LocalGroupMember {
param(
	[Parameter(ValueFromPipeline=$true)]
	[string[]]$ComputerName = '.',
	[Parameter(ValueFromPipeline=$false)]
	[string[]]$GroupName = 'Administrators'
)
<#
Get-WMIObject win32_group -filter "LocalAccount='True'" -computername $Server |
  Select PSComputername,Name,@{Name="Members";Expression={
   $_.GetRelated("win32_useraccount").Name -join ";"
  }}
Get-CimInstance win32_group -filter "LocalAccount='True'" -computername $Server |
Select PSComputername,Name,@{Name="Members";Expression={
 (Get-CimAssociatedInstance -InputObject $_ -ResultClassName Win32_UserAccount).Name -join ";"
}}

to find local admin group name:
$objSID = New-Object System.Security.Principal.SecurityIdentifier("S-1-5-32-544")
$objgroup = $objSID.Translate( [System.Security.Principal.NTAccount])
$objgroupname = ($objgroup.Value).Split("\")[1]
#>

PROCESS {
	foreach ($computer in $ComputerName) {
		foreach ($group in $GroupName) {
			$grp = [ADSI]"WinNT://$computer/$group,group"
			Write-Verbose ('Group: ' + $grp.Path) # test for existance
			$members = @($grp.psbase.Invoke("Members")) 
			$members | Foreach-Object {
				$name = $_.GetType().InvokeMember("Name", 'GetProperty', $null, $_, $null)
				$class = $_.GetType().InvokeMember("Class", 'GetProperty', $null, $_, $null)
				$parent = $_.GetType().InvokeMember("Parent", 'GetProperty', $null, $_, $null) -replace 'WinNT://' -replace '.*/'
				'' | Select-Object @{n='Class';e={$class}},@{n='Domain';e={$parent}},@{n='Name';e={$name}}
			} 
		}
	}
}
}

function Get-LocalGroup {
param(
	[Parameter(ValueFromPipeline=$true)]
	[string[]]$ComputerName = '.'
)
PROCESS {
	foreach ($computer in $ComputerName) {
		$comp = [ADSI]"WinNT://$computer"
		Write-Verbose ('Computer: ' + $comp.Path) # test for existance
		$comp.psbase.CHildren | Where-Object {  $_.SchemaClassName -eq 'group' } | Select-Object -ExpandProperty Name
	}
}
}