Grouping items is useful when you have two or more content boxes, and one might grow as a result of merged data or API data of different lengths. For example, you may have an API text box or a text box with merged data (perhaps containing your message text), followed by a signature box. If the message is very long, it might otherwise overlap the signature box. When you group the two boxes, it will keep them together, but they will not overlap.
To create the grouping, click the first box, then hold down the Shift key and left-click on the second box. Finally, click the Group icon in the upper left. You can ungroup them by clicking Ungroup. When two boxes are grouped, their content cannot be edited until they are ungrouped.