Yes, you can use the family group on 8 accounts regardless of which console they're on. I just got it and am sharing it with 7 other friends who can also play online on their consoles, and the accounts don't even need to be set to the same country (though they apparently do need to be in the same region).

The $20 individual membership is for one account only.

Edit: oh never mind, apparently they can even be in different regions based on post above.