Scene tree panel allows you to browse the hierarchy of the scene, change it and do some other things. You may drag and drop items to reorder or re-parent them, there is a context menu that helps to perform basic operations like cut, copy, paste, etc and also access Boxshot tools.
You may drag a node to another one to group them, so they move together. You may also create a Group node, using Scene → Create menu. That special node type is not visible at rendering time and useful to put some other nodes together.
Another option is to select the nodes you want to group, then right-click them, and select Tools → Group Selection in the context menu.