Those on the $15/month plan can share their Spotify subscriptions among six family members, who can use Premium on their own accounts. With the Voice Match feature, Home Mini can recognize who's speaking and personalize music playback for them.
It's a smart deal for both companies. It should snag Spotify some new subscribers, while Google could get more people hooked on YouTube Music Premium -- Home Mini comes with a three-month trial of that service. It might seem strange on the surface that Google has partnered with Spotify for this promotion when it has its own music streaming service, but perhaps it determined that getting its smart speaker and Google Assistant into more homes (and potentially luring YouTube Premium subscribers) is worth the tradeoff.