If you are rich enough to join, then you don't have to pay to be a member. That is the central idea behind what might be dubbed “the billionaire’s club,” a series of conferences for family offices and ultra-high net worth individuals that has been running for a decade.It's a lonely life being a billionaire. Seriously. There are not many of them and it is not like they can all bump into each other in the pub. Last year there were a total of 2,825 billionaires in the world, according to market research firm Wealth-X, the highest number ever. But that is barely enough people to fill a small football stadium, Billionaire Club or a large concert hall. If you have a billion dollars where do you go to hang out with your peers and swap gossip about how to deal with problems only the mega-wealthy have?Ten years ago Katja Muelheim and Tobias Prestel almost stumbled across the format that has since attracted billionaires like bees to honey without really meaning to, if you listen to the way Prestel tells it.