Every so often I get invited to an invite-only conference. Usually I decline, because the idea seems wrong. You don't know who is going to have the great idea that could break through and create new opportunity. Also, some of the best people I've met at my own conferences were people I didn't invite. Never would have thought to invite them because I didn't know them. A couple of them have become long-term close friends. How about that.
But, if you're going to discuss open formats and protocols, having an invite-only conference is not only a bad idea, it can be unethical. And can destroy the open-ness of the protocols you're discussing.
For example, the early FOO Camps discussed the future of open web formats. There were people there who felt they had a say in their future, and they're right about that -- they did have a say. But they didn't have an exclusive say. Not sure, but I think they even drafted some specs at these conferences.
Another example was a social web "summit" held last summer in Portland. Invite-only. Lots of people from Google, and other big companies, and also some independents. But there were some interested people (such as myself) who were conspicuously absent, un-invited. Wonder why? Doesn't matter. There was nothing open about this stuff, but they still pretend it was. Might as well start over, if being open is part of their strategy.
When I'm invited to one of these conferences, and want to go, even though it's not open-to-all, I ask the organizers to stipulate, to all participants, that open formats and protocols will not be discussed. This means that when the subject comes up, someone in the room can remind them that it's off-topic. And the integrity of all those involved and of the formats, can be protected.