Do you already know how to technically search for groups in Facebook?
I search for terms about my content. As I make tutorials about editing videos in free software (amongst my maker stuff) I searched for terms like "free video editing", "open source video creation", "openshot", "cinelerra" or "kdenlive" (where that letter three are different free video editing programs). From the results I joined the groups that looked the most promising (most members - but even small groups can work well if they have the right members). Note that I didn't join the groups because I make videos but because I offer something that the groups is hungry for - tutorials.
The reason why most video promotion communities don't work is that the people are not interested in your content. If they watch your videos then most likely just because they want to give you a favour. Such people are not really going to watch all your videos and subscribe in contrast to somebody who genuinely enjoys your content. So if you make pet videos join a community that is about pets and not about videos. If you make gaming videos join a community that likes gameplay and not a community that is about "promoting" videos. If you are making random vlogs this may mean that you need to join a different set of groups for every single video you make. So it could be yet another wise reason amongst others to develop a common theme for the video content on your channel.
What happens then depends on the rules and behaviour in each groups. Generally I start by an introductory post. In this case I stated that I have a YouTube channel, edit my videos with free software and that I also make tutorials about it. Then I start to constructively comment on some other people's posts. I don't post random stuff like "Nice video" or "Good stuff". Instead I try to be specific e.g. "I like how you solved the problem with the colour mismatch" or "Your puppy has such a fluffy fur. It is a lot of effort to comb?". I'd rather tend to stick to people who already got many answers as they seem to behave correctly and my answer will probably pop up in many notification feeds.
Only when I got a feeling about how people post in that community I start to create my own posts that contain links to my videos. You don't want to annoy people with mediocre quality. If you are putting a lot of effort to drag from a community you also want it to kick in. Don't post your everyday crap but only the best of the best with honours sir.
But you are not done yet. Try different groups at different times to see in Analytics which ones work and which ones don't. Views are not the most important criterion. Check for the audience retention and sort out communites where people tend to drop off early. You will most likely notice that these are the so called "promotion" groups. This will then finally lead to higher audience retention for future videos and make them rank higher in YouTube search and suggestions.
Hmm looks like it is about time for me to wrap this up for a tutorial ...
