Write an article based on this "Find the song you want to add to your collaborative playlist. Tap the ••• next to the song. Tap Add to playlist on the menu. Select your collaborative playlist on the list."
Open an artist, album or playlist, and find the song you want to add. You can find this button next to the song's name. It will open your options menu. This will open a list of all your available playlists. This will add the selected song to your collaborative playlist. Once you add a new song to a collaborative playlist on your device, it will instantly show up on your contact's app.