What's the best way format an Excel file to mimick Salesforce's grouping feature in reports?
Well, this is quite a late response ... but ... from looking at this: (since I'm not familiar with Salesforce) Help | Training | Salesforce -- I'd suspect that Pivot Tables might be what you're looking for in Excel! Pivot tables will let you group the content in whatever which way, and move your columns to rows & vice versa very easily with the panel it will display to the left.
Here's an article on that: Create a PivotTable to analyze worksheet data - Office Support
If you're looking for something more specific, let me know .... I love Excel.
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