You could use an actual ShapeNode as group box. Once the user draws it, set its Transparent property to hide the node's geometry, and call childNode.attachTo(groupNode) method to attach the nodes you find enclosed within the group's bounding rectangle. Now the child nodes will be drawn behind the group, and if the user tries to select any of them they will actually select the group node. Moving it will also move all attached nodes.

I hope that helps,
Stoyan

There's undocumented method in Diagram class used for lasso selection:
getIntersectingItems: function (rect, intersectingItems, multiple, ifIntersect)

Pass the bounding rectangle as first argument; an empty array that receives the items as second argument; true as third argument; set ifIntersect to true if nodes should be collected when they intersect the specified rectangle, or false if they should be entirely inside it.

I hope that helps,
Stoyan

There's only the diagram's nodeCreated event raised for all nodes, you'll have to add a case for the group box inside it. You could also replace that node's endDrag() function with your own version and use it as a handler.

I hope that helps,
Stoyan