Nessuna garanzia
string userPath = string.Format("WinNT://{0}/{1},user", domain, user);
string groupPath = string.Format("WinNT://{0}/{1},group", Environment.MachineName, group);
using (DirectoryEntry group = new DirectoryEntry(groupPath))
{
    group.Invoke("Add", userPath);
    group.CommitChanges();
}