I don't think there's any way to filter out the "sea of clueless groupthink" that you mention, which is the reason for the private forum. The privacy would give it a chance to develop a culture of its own, and effective reinforcement mechanisms to incentivize the behavior we'd want.

I also don't think there'd be any way to authenticate or filter the people who joined, other than an initial mutual agreement of the creators about what they want (probably a good first topic of discussion.)

Possibly the name of the group could work as a filter, though. I'd choose the name "Unashamed Capitalists". Every startup person should be a capitalist right? And shouldn't be ashamed, right? This is a bit broader so that people who are not yet "successful" would be welcome (newbies and pros are what I'm interested in, though the OP wanted a forum for successful people... my perspective is that the problem isn't newbies but the culture. I could be wrong.)

Given the way voting has happened in places like here (where a comment of mine that simply states two objectively verifiable facts has a negative score because those facts are inconvenient) and reddit and dig makes me wary of it.... however, it might work as a form of ad hoc curration. Some set of people deemed "successful" or "clued in" could do the voting, and maybe there'd only be an up vote.

Anyway, just my thoughts. I think the person who actually finds some software and sets something up will have the most influence... but I'm much more interested in a private forum with reasonable open admission policies than I am a public forum...