IMO a very intuitive feature is missing and it's really annoying to use the current method.
When I want to add a new notebook to an existing group, if I want to use the UI, I have to scroll to "Notebooks" click on it, then click the New Notebook button, then find the new notebook and drag it into my group/stack/whatever.
If I click Shift+Command+N I can create a new notebook, but even if I've selected a stack, it doesn't add the new notebook to it.
Let me right click on a stack, and then click a New Notebook on the pop-up menu. Have that notebook show up within the stack.