It's easy to see why YouTube Party is one of the most popular extensions for watching YouTube videos with friends. 

The basic idea behind the app is simple and effective: it allows users to watch videos and have the playlists synchronized across multiple devices. So, you can sit back, relax, and watch all your favorite videos on one device, and all of them will be synchronized with the ones on your friends'.

What you can do with the app is pretty simple.

You can add your friends to the app, watch videos together and chat about them. You can create your own watch party. You can add comments to a specific video, and chat about them. You can view a list of the videos of all the users who have the extension installed. You can leave a session and restart it again.