Not sure how known the concept is, but I think it is fairly well known, especially if you run a bootstrapped startup. And if you're listening to the podcast Startups For the Rest Of Us (recommended!), I'd say it's probably close to 100 percent. One way of finding people for your mastermind is to visit the MicroConf conference, run by the same people as the podcast. (It's a lot easier getting a ticket for the European conference than the US one.)