Once a group is created or selected, you can expand it by double-clicking on it while holding the CTRL key. To go back to the parent group, hit the BACKSPACE key. You can also use the menu items found under the "Grouping" menu in the contextual menu (right click on a group to access it).

Once inside a group, you will see a white text line at the bottom of the patchbay. This line identifies which group is currently expanded.