i purchased youtube premium family from the apple store but can't figure out how to add family. we are all already on the apple sharing plan

I had purchased the YOUTUBE premium for myself but everyone in my household uses YOUTUBE. I went into my subscriptions and upgraded to YOUTUBE PREMIUM FAMILY. But now, i can't figure out how to add my 2 boys to my family plan so they could use it too. I don't want to cancel my subscription and start over. I had this for a long time and have alot of videos i want to keep.