Add User to Local Group

$username = 'SomeUserName'
$localGroup = 'Administrators'

$members = net localgroup $localGroup
$result = $members | Select-String $username

if ($result.Count -eq 0) {
    Write-Host "Adding $username to $localGroup"
    net localgroup $localGroup $username /add
}
else {
    Write-Host "$username is already a member of $localGroup"
}

# ---------

Write-Host 'Adding accounts to local Administrator group on servers...' -ForegroundColor Cyan

Invoke-Command -ComputerName 'server_one', 'server_two' -ScriptBlock {
    $usernames = @('user_one', 'user_two', 'user_three')
    $localGroup = 'Administrators'

    foreach ($username in $usernames)
    {
        $members = net localgroup $localGroup
        $result = $members | Select-String $username

        if ($result.Count -eq 0)
        {
            Write-Host "Adding $username to $localGroup on $Env:COMPUTERNAME"
            net localgroup $localGroup $username /add
        }
        else
        {
            Write-Host "$username is already a member of $localGroup on $Env:COMPUTERNAME"
        }
    }
}