Hi Simon,
First, you need to make sure that the new row has the same value in the cell used for grouping. Then you need to take the index from the rows collection because the RowIndex property will return the index inside the group. Here is an example:
Detailed information about the indexes can be found here:
Rows vs ChildRows.
I hope this will be useful. Let me know if you have additional questions.
Regards,
Dimitar
Progress Telerik
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