How do I make a collaborative playlist on Spotify so friends can add their own songs?

Make the playlist first, then open it, tap the three dots (or the playlist menu) and choose 'Collaborative playlist' / 'Invite collaborators'. That generates a share link, and anyone you send it to can add, remove and reorder songs. Good news for the road trip: collaborators do NOT need Premium, a free Spotify account can add to a collaborative playlist. Everyone does need an account though, and you can tap a track to see who added it. Heads up: any collaborator can also remove songs, so it's a bit of a free-for-all. Fine for friends, less so for strangers.

Nobody needs Premium to join in, free accounts can add songs to a collaborative playlist, which is perfect for a group trip. They just need a Spotify account and the share link.