You will need to create a custom GroupMargin, in which you will do the usual overriding stuff, plus override the Borders, as you did in the row/cell derivation.
Then you will use this custom GroupMargin on the group that serves as a template to the GroupByRow GroupTemplate.
i.e.: GroupByRow.GroupTemplate = new Group( new myGroupMargin() );
Imported from legacy forums. Posted by André (had 5025 views)