Android: Several of Samsung's Android phones come with a cool feature called "Group Play" which lets you connect several phones to play the same song on all of them at the same time. If you aren't using a Samsung gadget, you can now do that with Chorus.
The app is easy to set up and lets you connect either devices through the same Wi-Fi network, or by creating a Wi-Fi hotspot from the "host" device. The host device is also the one that gets to decide (and save) the playlist, so make sure that's the one loaded with all the music. You can start playing the music immediately and as every new device connects, it will auto-sync the playlist.
In my experience, the auto-sync worked perfectly for the first two devices, but then I had to manually sync the third and fourth devices. When manually syncing, the song restarts — it's not a big deal though.
The app generally works just as advertised. The default music player isn't the best you'll get, but if you're out and need loud music, get everyone to download this app and you'll have an improvised loudspeaker. The number of devices is only limited by the connections allowed by your router or the device's hotspot.