Pasted

$w = Get-SPWeb  http://sp.acme.com
$u = $w.EnsureUser("i:0#.w|acme\user")
$g = $w.Groups["Administrators Group"]
$g.AddUser($u)
$g.Update()
 
# you can use also
# public void AddUser (string loginName, string email, string name, string notes);