Pretty simply, Spotify has changed their family plan pricing from a $17.99 for two accounts + extra $$ for each extra account to a flat $17.99 for up to 6 accounts.
If you already have a family plan, just add up to another 4 accounts or if not, might be a good time to merge them. If you're entrepreneurial, you could get 5 mates and pay $3.00 each!
Details on their blog page: https://news.spotify.com/au/2016/05/23/spotify-family-plan-g…
They must be trying to combat Youtube Red (which has free G Music).