You can add a button group this way:
1) Add a RibbonBar control to your form.
2) Add a RibbonBarChunk in some of the ribbon's command tabs by clicking the chunk which has a "New Chunk" label.
3) Right click in the chunk and select a "RibbonBarButtonGroup" from the context menu.
You can add buttons and other items in the button group and other button groups as well. By default the button group which is inserted directly in the chunk doesn't have border (the ShowBorder property is set to false). Each button group which is inserted in another button group has border by default (ShowBorder = true).
I hope this information helps.