Open Source Watch Videos Together Tools for Linux

Metastream allows you to watch internet videos with friends. It synchronizes media on websites so you can watch together.

Syncplay program allows you and your friend/partner/family member to watch same video file simultaniously on different PCs and different location by syncing playback of MPC-HC, mpv, mplayer2 and VLC players.

Jelly Party is a free Chrome & Firefox extension for watching Netflix, YouTube, Disney+ and many more streaming services remotely with friends — in sync. It features integrated text-, voice- and video-chat.

SyncWatch is a browser extension which lets you watch videos together with friends synchronously.