I'm not sure there is a way to get around your problem without doing something kludgy. Defining columns for a report affects the entire report. In other words, you can't have a grouping (whose header runs across the entire width of the page) and just have columns within that grouping.

I believe you'll have to simulate column behavior within the grouping with clever data manipulation.

Hopefully I'm wrong and someone else has come up with a brilliant solution to this one and will jump in with it.

--Brad