Two of the non-indexable communities I value most are not invite based.
I'm curious, the forums you frequent, are they based around a specific common interest? Acoustics, cooking, weightlifting, etc. ? Or are they general forums, kind of like hubski?
What's the best way to find a good forum? You say that your favorites are not googleable, so how would you find it in the first place?
I wish I knew the answer. The ones I'm fond of are actually built up around something else. My participation in them is based on affinity for something else. A community I'm no longer part of (but is apparently still going) actually predates google by a good 12 years.
I've never participated in one. I can't comment. EDIT: So a little clarification. I suppose that a Non-Disclosure Agreement is technically "invite based." However, I think there's a distinction there. The other thing to keep in mind is that technical communities have a reason to exist other than "shooting the shit" although that often becomes part of the fun. So I'm not really sure how to answer this question. On the one hand, I feel like I've never participated in one. On the other hand, the ones I've participated in are locked down tighter than The Illuminati but they don't feel invite-based (despite the 22-part web form I needed to fill out for one of them). I need a vacation.