A headphones button next to Play on Spotify on someone's Spotify activity plays the same track at the same position on your phone, then follows them: next tracks, skips and seeks, and pausing when they pause.
:stop: Tap it again to stop listening along. If Spotify isn't open, it opens Spotify and waits for it; it never starts music on an idle speaker or computer.
The button is dimmed on your own activity, or when no Spotify account is connected.
While you play something, the + menu next to the message box gets a Spotify button that invites the channel to listen along, sending what you typed as the message.
Needs a Spotify account connected to Discord and the Spotify app on your device. Spotify only allows it with Premium.
A port of the Add Listen Along and Send Spotify invite modules of nexpid's NexxUtils.