How do I set all items in a group or category to use the seasonality of that group as a whole?

You can control the seasonality settings for all your items in the Seasonality Settings screen in Forecast Manager.

 To have all items at or under a certain level of your forecast hierarchy, you can do so in one step by creating the seasonality settings at a higher level of the hierarchy and letting the settings inherit downwards in the hierarchy.  Let's say you have a hierarchy level called "Item Class", and a item class called "P-CBL":

1. Open Forecast Manager and click the Tree view.
2. Click on the P-CBL item class - the P-CBL data will load
3. Click the Seasonality Settings tab.
4. Set the index type to “Use Hierarchy Level”
5. Select “Item Class” in the “Use Level” box that appears
6. Click Save

 This seasonality setting will be propagated downwards to all the item-sites in that item class, so that each item will use the overall seasonality of the item class, rather than try to figure out their own.