How to Add Users into a Windows Group using PowerShell?

# Need to open PowerShell as admin
# The example is for a local account. It would be the same for a domain account


PS C:\Windows\system32> $objUser=[ADSI]("WinNT://Xu-Acer-PC/PSTest")
PS C:\Windows\system32> $objGroup=[ADSI]("WinNT://Xu-Acer-PC/Administrators")
PS C:\Windows\system32> $objGroup.PSBase.Invoke("Add",$objUser.PSBase.Path)


# The above is just for one computer, how about hundreds of computers for the user? Need the automation codes!!!