try below code
DirectoryEntry localMachine = new DirectoryEntry("WinNT://" +
Environment.MachineName);
DirectoryEntry admGroup = localMachine.Children.Find("administrators",
"group");
object members = admGroup.Invoke("members", null);
foreach (object groupMember in (IEnumerable)members) {
DirectoryEntry member = new DirectoryEntry(groupMember);
}