system
Given the following post, respond as if you're commenting on Reddit:
human
How difficult would it be to create a watch party service like this?

I've been rummaging the internet looking for a service with the following functionality but have come up short, leading me to believe it's either difficult to make or monetize: 

1. Must be able to stream synchronized video playback to group members, such as Netflix, Youtube, Vimeo, Prime Video, Hulu, HBO, or other websites with video. 
2. Can also stream video originally from video files on a member's computer or cloud storage, and distributed potentially through a relay server if their upload speeds are too slow to keep it high quality for all members. 
3. Independent options for volume and subtitles. One viewer may use low volume while another uses high, etc. 
4. All members able to play/pause and seek the video and it remains synchronized on other member's screens.
5. 1080p quality.
gpt
2 main areas of immediate concern: legal structure and operating cost.  

There have been quite a few services targeting this in the last decade that have closed due to lawsuits regarding their hooking into other platforms.
  
1080p at a decent bitrate will be gigabytes per user per stream and if the point is to alleviate poor connections that transit will not be public. You would then go to a client to client distributed model and be using some form of udp mesh topography which is where things go from napkin sketch to a month of architecture design.