If you'd like some elements to be grouped together so that they can be moved, edited or aligned / arranged together, first select the Arrow tool, and then hold down and drag over the section of elements that you'd like grouped together.
Once these elements are all highlighted together, then you can select the Group tool from the toolbox at the top of the page.
Note: the Group tool will remain greyed out and unable to be selected until more than one element has been highlighted.