Clear | Deletes the data from the GroupItem list in memory. | 
  GroupList | The list of top-level GroupItem objects. | 
  IsClear | Checks if the GroupingManager contains any data. True if there is no data. | 
  ItemList | The list of references to the members of the group. | 
  Refresh | Fetches the GroupList according to the Filter and Group values, overwriting the list in memory. |