Become part of the community you wish for! We’re over on Grobbulus Classic Era where we have one guild per faction. We manage to find new people to join us routinely!
I think it helps that we have a common goal: vanilla wow, and that’s it. We aren’t looking to progress into TBC or SoD or anniversary or whatever. We just love Era and forever vanilla wow. Given that, we’ve been able to attract a lot of like minded and mature players over to join us on our little realm.