Use the filter function to filter out the parts that you can want to work with, you can also use the sort function to sort the list according to the wanted criteria's (typical according to some forecast error measurement or all parts in a product family...) after filtering. To work at group level you start creating a filter that includes all the items that you want to group and then push the Group button in the top row.
The Forecast Part List will be filled with items matching your query setting