Group is a very special element to conjoin multiple elements. You could not find it from the element category, instead ForeUI auto generates it and wraps the current selected elements inside when you click the button in the floating tool pane.
Group is also used as the wrapper of elements that embedded into the container. For example, when you embed a Placeholder element into a Tabs element, a Group element will be generated and wrap the Placeholder element inside.
Different than other elements, group can not have its even handlers. It is because Group is supposed to help laying out the GUI, you may need to ungroup or regroup quite frequently. So it is not a safe place to define the behavior for a group, since its event handlers will disappear if you ungroup it. When you select a group and press Ctrl + D, you will not open then action editor, instead you will get this message:
Although it is not allowed to define event handler for the entire group, you can still define the event handler on its member element.