Groups are a simple way to organize your file, add extra transform space to an object, or create custom origin points.

To create a Group, activate the Group tool and add an empty Group to the stage. Now, drag and drop any objects you'd like onto the empty Group in the Hierarchy.

To active the Group tool, press G.

You'll notice that after nesting objects under a Group that the Group indicator disappears and the bounding box updates to include all objects added to the Group.