[samibe]

I have the custom group labeled as 'Instrument' and each instrument shown in a particular song as a separate entry in the field. I also filled out the genre data for each song. This makes it so that for me to find a group of songs for a specific instrument and genre I can go to the Instrument tab, select the instrument and the filter the list by genre or I can go to the Genres tab, select a genre and filter by instrument.

If you wanted to use the custom group for something else, you could assign instruments to the Collections field. Then to find a particular instrument/genre group you would go to the Collections tab, select the instrument and filter by genre or go to the Genres tab, select the genre and filter by collection (instrument).