I am trying to write a financial report by department. Each department has ~ 40 expense account of which a dozen are payroll related. How do I show each account individually except the payroll accounts which I would like to group as one amount?

you would need a way to determine payroll for the rest of the expenses, then create a group based on this criteria.  You don't need to display the group, or it can display as a blank line separating the parts of the report.