You can use ADSI.
If the current logged-on user doesnt have the necessary permissions, use the following replacing MyComputer with the name of the PC, Admin with a username who does have the necessary permissions, pass with the username's password.
Set oWinNT = GetObject("WinNT:"

Set oComputer = oWinNT.OpenDSObject("WinNT:MyComputerName,Computer","admin","pass",1)
If the current logged-on user does have the necessary permissions, use the following replacing MyComputer with the name of the PC.
Set oComputer = GetObject("WinNT://MyComputer,Computer"
Then, get the group and add the user:
Set oGroup = oComputer.GetObject("Group","Administrators"

If Not oGroup.IsMember("WinNT://yourdomain,youruser"

Then
oGroup.Add "WinNT://yourdomain,youruser"
End If
Jon Hawkins