Syncplay is a free tool which synchronises media players over the internet so that distant friends can watch movies together.
Setting up the program takes a little work. You'll have to manually find a server (public ones are listed on the Syncplay site) or set up one yourself. You'll need a compatible media player (VLC, mplayer2, MPC-HC, mpv). If the video isn't already available on a streaming URL then everyone who wants to watch must have a local copy of the file, and you must choose a room name and pass it on - along with the server name - to everyone who wants to watch.
This isn't as bad as it sounds, though. You'll figure it out in maybe five minutes. And once you've set everything up, your configuration can be saved for instant recall next time.
And once Syncplay is running, the program works very well. A "Notifications" panel lists users as they connect to the server, you can start play with a click - once everyone is ready - and anyone can pause playback, maybe step back a scene and resume, and everyone else will see the same results on their own system.
It could be easier to use, but once you've figured out the basics Syncplay is a straightforward way to synchronise your video viewing.