I have also heard that you can request to join a closed group also and that sometimes they let you join. I say it's worth the try if it is a good fit for you.
I have found some boards to join because of Facebook groups focused on Pinterest or Etsy sellers. If you search in Facebook "Pinterest Group Board" you'll find a bunch and see what group seems like a good fit.
I also do Tailwind Tribes and have success with it.
I also started my own Facebook group just for Etsy/Pinterest users in my state. We are only at 89 members but it's good group who actually engage with each other.