As of Q3 2012 we have introduced a new property of the RadGridView control - GroupRenderMode. It has two possible values:
Nested: This is the default value and it will nest GridViewGroupRows into one another. With this group render mode every group is represented by its own UI container and groups are nested in a hierarchical manner. The visual element representing the grouped row is GridViewGroupRow.
Flat: This mode represents our new virtualization mechanism with a completely new item container generator. It simply renders rows one below the other and all rows are on the same level. The visual element representing the grouped row is GroupHeaderRow.
For compatibility reasons, the Nested render mode is also preserved, but it is highly recommended to use the new Flat mode as it leads to better performance and most of the latest features and bug fixes only target this mode.
Please note that when you use the Flat mode, you should work with GroupHeaderRow, not GridViewGroupRow.