I have a piece of VBS code that I have modified that basically adds a specified domain user and group to the PCs local Administrators group. It works on Windows 7, but not on Windows 8.

Call AddUserToGroup("./Administrators", "myDomain.net/NetworkAdminis")
Call AddUserToGroup("./Administrators", "myDomain.net/Domain Admins")
Call addDomainUser("myDomain", "myUserGroup")


Sub AddUserToGroup(local, domain)
	On Error Resume Next

	Dim objLocalGroup
	Dim objDomainGroup
	Dim server
	
	For Each server in servers
		Set objLocalGroup = GetObject("WinNT://" & local & ",group")
		Set objDomainGroup = GetObject("WinNT://" & domain & ",group")
		With objLocalGroup
			.Add(objDomainGroup.AdsPath)
			.SetInfo
		End With			
	Next

	Set objLocalGroup = Nothing
	Set objDomainGroup = Nothing

End Sub


Sub addDomainUser(strDomain, strUser)
   
    Dim strComputer
    Dim objWshNet
    Dim objGroup
    Dim objUser
    
    Set objWshNet = CreateObject("WScript.Network")
    strComputer = objWshNet.ComputerName
    Set objGroup = GetObject("WinNT://" & strComputer & "/Administrators,group")
    Set objUser = GetObject("WinNT://" & strDomain & "/" & strUser & ",user")
    If Not objGroup.IsMember(objUser.ADsPath) Then
        objGroup.Add (objUser.ADsPath)
    End If
    
    Set objWshNet = Nothing
    Set objGroup = Nothing
    Set objUser = Nothing
    
End Sub