Thank you; I had never heard the term mastermind in that context before. Is it a well-known concept? The second result when googling "startup mastermind" is the post you mention, and the first one is... a reddit thread pointing to that same post.
But the idea is exactly what I would need, so thanks!!
Glad to help. I've been in one for about a year or so, and can't recommend it enough.
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.)
(I'm not sure how universal "mastermind" is, so here's a link to help explain it a bit, and how to run one: http://www.startupsfortherestofus.com/episodes/episode-167)