You send them a note, asking to join. And then when you submit a dev, the club features it too; so those people watching the club will see your dev in their deviantWATCH even if they don't watch you personally.
As for the invitation... hmm... My personal advice to you about nekoyaoishotta is: wait a few weeks more. If the membership grows beyond 20 people, then I think it's safe to join.
You can submit older works to the clubs too, once your membership is approved.