With SyncWatch you can watch videos and listen to music from YouTube synchronously with your friends. Just create a room and start watching! SyncWatch is free and no login is required.
Kast Alternatives
Kast is described as 'Social sharing platform where users can stream content from their computer to the people in their channel (called parties). Kast parties allow up to 20 people to share their screens and up to an additional 100 people to view in the party' and is a watch videos together tool in the social & communications category. There are more than 25 alternatives to Kast for a variety of platforms, including Web-based, Mac, Windows, Linux and Google Chrome apps. The best Kast alternative is Metastream, which is both free and Open Source. Other great apps like Kast are Hyperbeam, Syncplay, Kosmi and OpenTogetherTube.