Group calendars provide specific views of the calendar not possible using just the normal calendars. They can also show multiple calendars on the same window.
The group calendars are accessed by clicking on the individual button on the toolbar for the group calendar you want (Mac) or by clicking on the View button (Windows) and selecting the group calendars in the bottom portion of the list.
In most group calendars some of the filters will also apply.
Below we'll go through each window in detail.