Use this property to set the group to which an item belongs. You can also set the group in the ListViewItem.#ctor constructor, or you can use this property to modify the group membership at run time. If you set this property to null and there are groups in the ListView.Groups collection, the item will appear in the default group, which has the header label "DefaultGroupSystem.Windows.Forms". The default group is not contained in the ListView.Groups collection, and cannot be altered. It is primarily useful in debugging to ensure that all items have been properly added to groups.