Run the server on one phone, then the app as a client on many devices to play music with Spotify Connect - all at exactly the same time.
SnapCast is a multi-room client-server audio player, where all clients are time synchronized with the server to play perfectly synced audio. It's not a standalone player, but an extension that turns your existing audio player into a Sonos-like multi-room solution.
This app has built-in capability to act as client and/or server. You may need to set the latency for each phone in order to get perfect synchronisation.
The server's audio input is a named pipe /tmp/snapfifo. All data that is fed into this file will be send to the connected clients. One of the most generic ways to use SnapCast is in conjunction with the music player daemon (MPD) or Mopidy, which can be configured to use a named pipe as audio output.
For more information, see the website at https://github.com/badaix/snapcast