For my public space on the internet, I wanted to create a social space for sharing music like turntable.fm (RIP). Although this service was shut down in 2013, I was lucky to find another web service which has since come online to offer similar features, JQBX. JQBX allows you to create public or private rooms where users can take turns as virtual DJs with music streaming from their Spotify account.
The room I created for ITP is here: https://app.jqbx.fm/room/itp-jukebox
I invited a few people to join through the unofficial ITP slack community. I would say it's mostly public, but still has the barrier of entry of requiring Spotify login. It's easy to get a free trial of Spotify, but some people may have already run through their 30 day trial, which is a constraint that can be overcome but requires tedious workarounds.