So here's a trick I learned in case someone is having trouble adding an inherited windows form control to a derived class' group box.

You'll find if you try to drag/drop the inherited control, it won't actually be addable to the group box.

So the trick here is, while you're dragging with the left mouse button into the group box, click and release the right mouse button, and you'll notice the control is now a child of the group box.

That's all there is to it.