When you tap on the new music option, you'll get a sheet that shows you the last few tracks you've played, a search field to find more music, and a spot to start creating a group playlist. You can share any track you find here to the conversation, whether your conversational partner is a Spotify member or not. Any recipient can tap a little play triangle to preview the song, and you'll be able play it via the Spotify app if you have it installed. If you don't like this feature, you can turn off M Suggestions at any time in your Profile settings — but it will turn them all off, not just Spotify.