I thought I was going crazy - the Google posts talking about collaborative playlists are pretty recent, but I spent 30 minutes clicking all over the place in Spotify in the Chromebook "app" (I guess that's just a fancy button to take you to the default webplayer?). 

Finally, frustrated, I go to the app on my phone, and sure enough in the playlist dropdown there's "Make Collaborative".

WHAT?!

I don't know if I'm upset at Spotify or this useless Chromebook, but come on now. I pay for this. Just add a button, please.

Also: can I not drag songs into a playlist? I hope I'm just an idiot and don't know how to use this. I really hope.