A lot easier than it sounds, you are able to unlock this trophy by simply selecting new game on a prison to play in the main menu, selecting a new save and setting the "playmode" for the prison (at the bottom right before you start it) to public (instead of local or private) and press play.
(If it doesn't unlock after doing that don't worry, since sometimes the trophies take a minute to register It "should" unlock if you back out and repeat the process, I didnt need anyone to join my game to unlock this trophy).
Hope this guide helps clear up any potential confusion you would have upon reading the trophies description (for those that may think "classic game online" is similar to play versus or something).