Since the 4.3.0 release, the playlist sharing has been added to Plexamp (thanks a lot btw).
From my account (plex owner), I can share playlist to my home users and they can see them.
But for some reason, none of my home users can share playlist. The option is missing on Plexamp for them (everyone is on the 4.3.0 version).
Thanks for your answer. That explains everything but makes the feature almost useless then.
It would be better that home users could also share playlists.
For example, I can share playlists with my wife which is a home user but she cannot share hers with me.