A simple way to group is to select the elements that you want grouped together. Then press "group" from the Advanced button. 

Another way to group elements is to create the group (e.g. Section 3) first, then put the desired elements under "children" in the JSON.

In the example below we "Edit JSON" for Q and add a new element "Section 3" under the list of children. Then we copy the elements we want in Section 3 to the clipboard. The elements are deleted from Q 's list of children so it won't be referenced there any longer. Then we "Edit JSON" for Section 3 and paste in the elements under the list of children. Section 3 is no longer empty, and it's referencing the correct elements.