One example: https://wiki.neuromatch.social/Monsterdon
This is a weekly Mastodon watch party 6PM PT every Sunday. You will need Mastodon or a Fediverse software capable of using hashtags. You start watching the movie at that time and you filter for the Monsterdon hashtags, so that people's chatter is roughly synced up to the point of the movie you are watching.
If you're talking about other kinds of watch parties you'll need to be more specific. Like just having a group of friends over or doing something with B-movie lovers from your local community, or other social media watch partiess will depend on the group.