We'd need to know more about the view you are trying to build and the data you have to work with. I do know that groups can be used on the Level of Detail shelf. If you can post a packaged sample workbook (.twbx) or describe the problem you are approaching in more detail, someone will certainly try to help you.
Thanks Matthew. I spent a minute trying to get a sample together, and then it clicked...
For anyone reading this, I was trying to build a budget vs. actual comparison by using a third data source as the scaffolding for the budget numbers and the actual numbers. I had created the groups within the scaffolding data source; this was incorrect. After thinking about it, I went back and created the groups within the Budget data source and the Actual data source, and then switched the primary-secondary relationships from "City" to "City (Group)". The scaffolding correctly pulled the new groupings.
Great, glad it worked out!