Creates a new group summary item with default settings and adds it to the end of the collection.
A GridGroupSummaryItem object representing the new summary item.
You should modify property values of the object obtained to customize the summary item as needed. Alternatively, you can use another Add method's overload that specifies the new summary item's settings as parameters.
If you need to add several group summary items to the collection, consider using the GridSummaryItemCollection.BeginUpdate and GridSummaryItemCollection.EndUpdate methods to prevent the collection from being updated until all required items are added. Alternatively, you can use the GridSummaryItemCollection.AddRange method, which adds a predefined array of summary items.
To remove a group summary item from the collection, use the GridSummaryItemCollection.Remove method.