There's no secret there, there's just a gatekeeper you have to get past, and it's a little bit different for every one. Some will let you right on through, but usually you get to know the board owner really well, interact with them a bunch on Pinterest, and ask for an invite. You can also create generic Pinterest accounts and use a bot to load them up with very similar content to what they're looking for with a couple search functions, but that's probably not worth explaining on this forum.

Also, getting on the community boards is only the beginning of the battle.