How do you group by week in Excel PowerPivot?
The most common approach I've seen is to use a calendar table. Setting up the calendar table can be a little tricky, but once you have it in the data model, join the calendar table to the sales table on the date. Then you can slice and dice and group by date dimensions such as year, month, week, etc.
Here's a good video that walks you through setting up a basic calendar table. https://www.youtube.com/watch?v=c4ezRLg31hc