Code

public void GroupNodes()
        {
            if (DesignDiagram.Selection.Nodes.Count > 1)
            {
                var groupNode = DesignDiagram.Factory.CreateShapeNode(0, 0, 1, 1);
                groupNode.ZBottom(false);
                groupNode.Transparent = true;

                groupNode.HandlesStyle = HandlesStyle.Custom;
                groupNode.Tag = "Group";
                CreateGroup(groupNode, DesignDiagram.Selection.Nodes);
                foreach (DiagramNode child in groupNode.SubordinateGroup.AttachedNodes)
                {
                    child.Locked = true;
                    child.DataContext = groupNode;
                }

                DesignDiagram.Selection.Change(groupNode);
            }
        }
 private void CreateGroup(DiagramNode node, DiagramNodeCollection children)
        {
            Rect r = Rect.Empty;
            foreach (DiagramNode child in children)
            {
                Rect cb = child.Bounds;
                r = r.IsEmpty ? cb : Rect.Union(r, cb);
            }
            double margin = 2;

            r.Inflate(margin, margin);
            node.Bounds = r;
            node.SizeChanged += node_SizeChanged;
            node.MouseEnter += currentSelectNode_MouseEnter;
            node.MouseLeave += currentSelectNode_MouseLeave;
            node.Expandable = false;
            foreach (DiagramNode child in children)
            {
                child.AttachTo(node, AttachToNode.TopLeft);
            }
        }