<#
.SYNOPSIS
    Adds (or removes) domain groups in the local Administrators group on remote computers.
.DESCRIPTION
    Uses PowerShell remoting to run Add-LocalGroupMember or Remove-LocalGroupMember on each
    computer. The built-in Administrators group is found by its well-known SID (S-1-5-32-544),
    so it works on non-English Windows too. Each computer and member gets its own result row,
    and one unreachable machine doesn't stop the rest. Supports -WhatIf.
.PARAMETER ComputerName
    Computers to change. Accepts pipeline input, so Get-Content .\computers.txt | works.
.PARAMETER Member
    Domain groups (or users) to add or remove, e.g. CONTOSO\Tier2-Workstation-Admins.
.PARAMETER Remove
    Remove the members instead of adding them.
.PARAMETER Credential
    Alternate credentials for the remoting connection.
.EXAMPLE
    Get-Content .\computers.txt | .\Set-LocalAdminMember.ps1 -Member 'CONTOSO\Tier2-Workstation-Admins' -WhatIf
.EXAMPLE
    .\Set-LocalAdminMember.ps1 -ComputerName PC-0142, PC-0187 -Member 'CONTOSO\Old-Helpdesk' -Remove
#>
[CmdletBinding(SupportsShouldProcess)]
param(
    [Parameter(Mandatory, ValueFromPipeline, ValueFromPipelineByPropertyName)]
    [Alias('Name', 'DNSHostName')][AllowEmptyString()][string[]]$ComputerName,
    [Parameter(Mandatory)][ValidatePattern('^[^\\]+\\[^\\]+$')][string[]]$Member,
    [switch]$Remove,
    [pscredential]$Credential
)

begin {
    $verb = if ($Remove) { 'Remove' } else { 'Add' }
    $prep = if ($Remove) { 'from' } else { 'to' }

    # Runs on the target computer. Returns one result per member.
    $remoteBlock = {
        param([string[]]$Members, [bool]$Remove)
        $adminsSid = 'S-1-5-32-544'
        foreach ($m in $Members) {
            try {
                if ($Remove) { Remove-LocalGroupMember -SID $adminsSid -Member $m -ErrorAction Stop }
                else         { Add-LocalGroupMember -SID $adminsSid -Member $m -ErrorAction Stop }
                [pscustomobject]@{ Member = $m; Result = 'Done'; Detail = '' }
            }
            catch [Microsoft.PowerShell.Commands.MemberExistsException] {
                [pscustomobject]@{ Member = $m; Result = 'Skipped'; Detail = 'Already a member' }
            }
            catch [Microsoft.PowerShell.Commands.MemberNotFoundException] {
                [pscustomobject]@{ Member = $m; Result = 'Skipped'; Detail = 'Not a member' }
            }
            catch {
                [pscustomobject]@{ Member = $m; Result = 'Failed'; Detail = $_.Exception.Message }
            }
        }
    }
}

process {
    foreach ($computer in $ComputerName) {
        $computer = $computer.Trim()
        if (-not $computer) { continue }

        $approved = @($Member | Where-Object { $PSCmdlet.ShouldProcess($computer, "$verb $_ $prep local Administrators") })
        foreach ($m in $Member | Where-Object { $approved -notcontains $_ }) {
            [pscustomobject]@{ ComputerName = $computer; Action = $verb; Member = $m; Result = 'WhatIf'; Detail = '' }
        }
        if (-not $approved) { continue }

        $invoke = @{ ComputerName = $computer; ScriptBlock = $remoteBlock; ArgumentList = @($approved, [bool]$Remove); ErrorAction = 'Stop' }
        if ($Credential) { $invoke.Credential = $Credential }
        try {
            foreach ($r in Invoke-Command @invoke) {
                [pscustomobject]@{ ComputerName = $computer; Action = $verb; Member = $r.Member; Result = $r.Result; Detail = $r.Detail }
            }
        }
        catch {
            # Couldn't reach the machine at all: report every member as failed.
            foreach ($m in $approved) {
                [pscustomobject]@{ ComputerName = $computer; Action = $verb; Member = $m; Result = 'Failed'; Detail = $_.Exception.Message }
            }
        }
    }
}