When you subscribe to a calendar, each subscription must be added separately to your calendar application. While this provides some benefits, such as having the subscriptions in different colors, there is also some overhead in managing them.
A calendar group resolves this issue. It is a collection of calendar subscriptions. It is subscribed only once, and you can modify its content at WebCal.Guru.
A calendar group can contain any number of subscriptions, as long as there is more than one.
You can add calendar subscriptions to a group by dragging them (on a computer), or by clicking on the Edit button beside a calendar subscription and then selecting the group to which you wish to move the subscription to.
If you delete a calendar group, it will not delete the calendars inside it. Those calendars will be moved back to single calendars, where they can be removed if wanted.
One subscription can be only in one group at a time.