Add a domain user or group to local administrators

I tried to make this script as simple as possible for day-to-day use. This script takes three parameters:

• ObjectType: Type of object that you want to add to the local administrators group. ObjectType should be either User or Group.
• ObjectName: Name of the domain object that you want to add. ObjectName should be in the format DOMAINNAME\UserName or DOMAINNAME\GroupName.
• ComputerName: List of computer names on which you want to perform the operation. When no computer account is specified, the script tries to execute the action against the local computer from which you are running the script.
• Output:Out put will be saved to C:\Temp\<Result.CSV> with filename

<#

    .Synopsis
        Adds a user or group to local administrator group

    .Description
        This scripts adds the given user or group to local administrators group on given list of servers.

    .Parameter ComputerName
        Computer Name(s) on which you want to add user/group to local administrators

    .Parameter ObjectType
        This parameter takes either of two values, User or Group. This parameter indicates the type of object
        you want to add to local administrators

    .Parameter ObjectName
        Name of the object (user or group) which you want to add to local administrators group. This should be in
        Domain\UserName or Domain\GroupName format

    .Example
        Set-LocalAdminGroupMembers.ps1 -ObjectType User -ObjectName "AD\TestUser1" -ComputerName srvmem1, srvmem2

        Adds AD\TestUser1 user account to local administrators group on srvmem1 and srvmeme2

    .Example
        Set-LocalAdminGroupMembers.ps1 -ObjectType Group -ObjectName "ADDomain\AllUsers" -ComputerName (Get-Content c:\servers.txt)

        Adds AD\TestUser1 Group to local administrators group on servers listed in c:\servers.txt
    .Notes
        Author : Jeevan Bobba
        WebSite: www.vmwareenterprise.com

#>
[CmdletBinding()] Param(
    [Parameter(Mandatory=$true,Position=1)]     [ValidateSet(“User”,”Group”)]     [String]     $ObjectType,

    [Parameter(Mandatory=$true,Position=2)]     [ValidateScript({($_.split(“\”).count -eq 2)})]     [string]$ObjectName,

    [Parameter(Position=3)]     [String[]]$ComputerName=$env:COMPUTERNAME
)

#Name and location of the output file. Change this line if you want to alter the location
$ResultsFile = "c:\temp\Results.csv"
$ObjDomain = $ObjectName.Split("\")[0] $ObjName = $ObjectName.Split("\")[1] $ComputerCount = $ComputerName.Count
$count = 0
Add-Content -Path $ResultsFile -Value "ComputerName,Status,Comments"
foreach($Computer in $ComputerName) {
    $count++
    $Status=$null
    $Comment = $null
    Write-Host ("{0}. Working on {1}" -f $Count, $Computer)
    if(Test-Connection -ComputerName $Computer -Count 1 -Quiet) {
        Write-Verbose "$Computer : Online"
        try {
            $GroupObj = [ADSI]"WinNT://$Computer/Administrators"
            $GroupObj.Add("WinNT://$ObjDomain/$ObjName")
            $Status = "Success"
            $Comment = "Added $ObjectName $ObjectType to Local administrators group"
            Write-Verbose "Successfully added $ObjectName $ObjectType to $Computer"
        } catch {
            $Status = "Failed"
            $Comment = $_.toString().replace("`n","").replace("`r","")
            Write-Verbose "Failed to add $ObjectName $ObjectType to $Computer"
        }

        Add-Content -Path $ResultsFile -Value ("{0},{1},{2}" -f $Computer,$Status,$Comment )    

    } else {
        Write-Warning "$Computer : Offline"
        Add-Content -Path $ResultsFile -Value ("{0},{1}" -f $Computer,"Offline")
    }

}

Source  Thanks Sitaram