Who can be in a Family Sharing group?

I am the organizer of a family sharing group that includes my wife and son. Since we have some slots left, I am thinking of adding my parents, so they don’t need to get their own subscriptions on their set income. But they don't live with us, so I’m not sure if that’s an issue. Is anyone aware of any restrictions that would make that a problem? I haven’t found anything, but I want to be sure. Thanks!