Best is to go for the big guys in your niche and most likely they are in some good groups. Ask a few to invite you, 3/10 will say yes and invite you. I myself invited smaller accounts in groups I created this way.
Other way is to create one yourself, invite-only big accounts that are interested, and then, pitch them the account, once 1/2 join its easy to get others to join. I created few groups this way where I was the smallest account and then you can also remove people who don't engage etc..