It is a little more involved since the GridGroupingCOntrol can have other things besied Records visible.
Element el = this.gridGroupingControl1.Table.DisplayElements[rowIndex];
if(el is GridRecordRow)
{
GridRecordRow recRow = el as GridRecordRow;
int position = this.gridGroupingControl1.Table.UnsortedRecords.IndexOf(recRow.ParentRecord);
Console.WriteLine(position);
}