Currently, it's impossible to drag-and-drop a category under a group directly at its last position, because it needs to be put at least one place above the last group's child category to be moved inside the group. This is a tricky UX issues, but many websites (and game engines, which use hierarchical node tree a lot) solve it by detecting when the dragged item is just below the group's last child, and not too far (some UI show a horizontal bar that is more narrow when placing under a parent, but in your case the "Move to group (group name)" text should be enough), so you could do something similar.