Transform groups are essentially nulls that are parents of a hierarchy. By default, the transform group null is not visible in a 3D view: to make it visible, click the eye icon in a viewport's title bar and select Transform Groups.
To create a transform group: Select or branch-select objects and choose Edit Create Transform Group from the main command panel.
To display: Select a transform group null and press Enter.
A shadow is a visual reference that makes it easier to see and select the null. When you select the shadow, the null is also selected.