Thanks for this! My husband has been trying to join our family account for like years (I didn't even know he wasn't part of it and that we were overpaying my $10 a month until 6-8 months ago), and couldn't b/c he signed up with a Facebook account (back in 2011, when Spotify required that for new US users who weren't like me and already had accounts because of ...reasons) and support's official suggestion was to open a new account and migrate playlists, which just sucks.

Hopefully this works!