A while back when I was more UI focusssed, and the web was nice as it is now, I wrote a WPF diagramming tool (using MVVM).
This was quite popular and looked like this
Thing is a lot of people wanted it to support grouping. Someone took on this noble task, and sent me a nice PR for that which I have merged. So it now supports grouping/ungrouping too : https://github.com/sachabarber/MVVMDiagramDesigner
Thanks “vRage” for the PR