PowerShell - Add AD User/Group to Local Administrator Group

<#
.SYNOPSIS  
Script to add an AD User or group to the Local Administrator group
    
.DESCRIPTION
The script can use either a plaintext file or a computer name as input and will add the trustee (user or group) as an administrator to the computer
    
.PARAMETER InputFile
A path that contains a plaintext file with computer names
.PARAMETER Computer
This parameter can be used instead of the InputFile parameter to specify a single computer or a series of
computers using a comma-separated format
    
.PARAMETER Trustee
The SamAccount name of an AD User or AD Group that is to be added to the Local Administrators group
.NOTES  
Name: Set-ADAccountasLocalAdministrator.ps1
.EXAMPLE  
.\Set-ADAccountasLocalAdministrator.ps1.ps1 -Computer Server01 -Trustee MananChoksi
Description:
Will set the the JaapBrasser account as a Local Administrator on Server01
.EXAMPLE  
.\Set-ADAccountasLocalAdministrator.ps1.ps1 -Computer 'Server01,Server02' -Trustee Contoso\HRManagers
Description:
Will set the HRManagers group in the contoso domain as Local Administrators on Server01 and Server02
.EXAMPLE  
.\Set-ADAccountasLocalAdministrator.ps1 -InputFile C:\ListofComputers.txt -Trustee User01
Description:
Will set the User01 account as a Local Administrator on all servers and computernames listed in the ListofComputers file
#>
param(
     [Parameter(ParameterSetName='InputFile')]
     [string]
         $InputFile,
     [Parameter(ParameterSetName='Computer')]
     [string]
         $Computer,
     [string]
         $Trustee
)
<#
.SYNOPSIS
     Function that resolves SAMAccount and can exit script if resolution fails
#>
function Resolve-SamAccount {
param(
     [string]
         $SamAccount,
     [boolean]
         $Exit
)
     process {
         try
         {
             $ADResolve = ([adsisearcher]"(samaccountname=$Trustee)").findone().properties['samaccountname']
         }
         catch
         {
             $ADResolve = $null
         }
        if (!$ADResolve) {
             Write-Warning "User `'$SamAccount`' not found in AD, please input correct SAM Account"
             if ($Exit) {
                 exit
             }
         }
         $ADResolve
     }
}
if (!$Trustee) {
     $Trustee = Read-Host "Please input trustee"
}
if ($Trustee -notmatch '\\') {
     $ADResolved = (Resolve-SamAccount -SamAccount $Trustee -Exit:$true)
     $Trustee = 'WinNT://',"$env:userdomain",'/',$ADResolved -join ''
} else {
     $ADResolved = ($Trustee -split '\\')[1]
     $DomainResolved = ($Trustee -split '\\')[0]
     $Trustee = 'WinNT://',$DomainResolved,'/',$ADResolved -join ''
}
if (!$InputFile) {
     if (!$Computer) {
         $Computer = Read-Host "Please input computer name"
     }
     [string[]]$Computer = $Computer.Split(',')
     $Computer | ForEach-Object {
         $_
         Write-Host "Adding `'$ADResolved`' to Administrators group on `'$_`'"
         try {
             ([ADSI]"WinNT://$_/Administrators,group").add($Trustee)
             Write-Host -ForegroundColor Green "Successfully completed command for `'$ADResolved`' on `'$_`'"
         } catch {
             Write-Warning "$_"
         }   
     }
}
else {
     if (!(Test-Path -Path $InputFile)) {
         Write-Warning "Input file not found, please enter correct path"
         exit
     }
     Get-Content -Path $InputFile | ForEach-Object {
         Write-Host "Adding `'$ADResolved`' to Administrators group on `'$_`'"
         try {
             ([ADSI]"WinNT://$_/Administrators,group").add($Trustee)
             Write-Host -ForegroundColor Green "Successfully completed command"
         } catch {
             Write-Warning "$_"
         }       
     }
}

Comments
Save above powershell in to Set-ADAccountasLocalAdministrator.ps1
Run this powershell command with pass parameters
.\Set-ADAccountasLocalAdministrator.ps1 –Computer computerName –Trustee domaine\userID