When it comes to creating a website, forum, blog, or community never be afraid to reach out to people with a lot more exposure than you. My largest website gained tractions fast, with 15k members in less than 6 months. One of the largest reasons was I hooked two celebrities in the field to become staff, and promote the site on their social media. One having over 16k subscribers at the time, as well as other people with 8k - 10k subs I also had join and promote it temporarily. Remember, the worse thing they can do is say no if done right.
When sending messages to individuals make sure you're not spamming them, or advertising at all. Instead simply reach out and say something like "Hello {username}, I'm currently working on a (website, blog, forum, community) which has some cool features I think will help out the community of Xtopic. I was wondering if you'd like to look into it and possibly join it." In this message there's no links, no forced promotion, just genuine discussion to see if the individual is interested in it. At this point you'll more than likely get 1 of 2 responses or no response at all. a "No, I'm not interested, thank you.", "Sure, I'd like to check it out. What's the link", or as stated no response. Either one of these are fine as you're simply marketing yourself in a way that isn't forced, or goes against any ToS 9 times out of 10. Granted, some sites may have more strict policies than others, however for the most part this should work.
When it comes to social media almost always there's a way to contact the individual, youtube you have the option to get their email if they have one which most youtubers do. Twitter, and most others rely on private messages, or public messaging to get the point across. Either way is fine. If anything you may have them join, from there you can try and talk your way into seeing if they'd be willing to do some sort of collab, featured video, etc. promoting your site.
