Metastream - Watch streaming media with friends
Clappr - An extensible media player for the web
Watch2Gether - Watch2Gether is an app that allows multiple people to watch the same content simultaneously and interact with each other while watching.
JW Player - JW Player is an embeddable media player.
StreamParty - StreamParty is a watch communiry, that allows you to watch video streams in sync with the people you care about, on multiple services.
IINA - The modern video player for macOS