You can programmatically add your group initially adding a group descriptor. Later when the user tries to ungroup , you may prevent this by handling the Grouping event such as :
sender, GridViewGroupingEventArgs e)
(e.Action == GroupingEventAction.Remove)
*You will need to add a check for e.GroupDescriptor in order to cancel the ungroup action only for the relevant group descriptor and leave the user ungroup by the rest.
In case you meet troubles implementing do not hesitate to contact me.
the Telerik team
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