Every phone in the room, one speaker.
The host presses play. Every browser streams the same track from YouTube and starts on the same millisecond, because they all agree on one clock. Skip a song and they all skip together.
Paste
Drop in a YouTube or Spotify link, or just search. Spotify links are matched on YouTube so everyone hears the same source.
Scan
Everyone joins from a QR code or a four-character code. Their browser is the speaker.
Cue
Each device buffers, counts down together, and hits play on the same beat. Drift is corrected while it plays.