Hi Stoyan,
I have tried GetPosition method of the System.Windows.Input.Mouse class. When I select some nodes and move them into Group, I get mouse point of that place, where I released the selected nodes.I have converted this mouse position to relative diagram position using ClientToDoc method;

Now when I going to check how many nodes are there in this mouse position, I am getting only one Node Instead of two. First one should be the selected node and the second one should be that group node which is behind this selected node.

I hope, now you have the clear idea about problem. Please suggest something to allow multiple items Drop in a group and vice versa.

Thanks,
Bala

Stiil unable to get two nodes at this position.
Mouse Cordinates are also (0,0).