Code

private void OnGroupNodes(object sender, RoutedEventArgs e)
{
      var groupContainer = diagram.Factory.CreateShapeNode(diagram.Selection.Bounds);
      groupContainer.Transparent = true;
      groupContainer.ZBottom();

      foreach (DiagramNode node in diagram.Selection.Nodes)
            node.AttachTo(groupContainer, AttachToNode.TopLeft);
      foreach (DiagramNode node in groupContainer.SubordinateGroup.AttachedNodes)
      node.Locked = true;
}

private void OnUngroupNodes(object sender, RoutedEventArgs e)
{
      var groupContainer = diagram.ActiveItem as ShapeNode;
      if (groupContainer != null &&
            groupContainer.SubordinateGroup != null)
      {
            var attachedNodes = new List<DiagramNode>();
            foreach (DiagramNode node in groupContainer.SubordinateGroup.AttachedNodes)
                  attachedNodes.Add(node);
            foreach (DiagramNode node in attachedNodes)
            {
                  node.Locked = false;
                  node.Detach();
            }
            diagram.Nodes.Remove(groupContainer);
      }
} 

Code

groupContainer.SubordinateGroup.AutoDeleteItems = true;
 

Code

if (e.Node is ShapeNode && e.Node.SubordinateGroup != null)
      foreach (DiagramNode node in e.Node.SubordinateGroup.AttachedNodes)
            node.Locked = false;