As you have mentioned Wagyu Mafia is a membership only restaurant. Information indicates that need to be a member of a culinary club run by the owner of the restaurant, the membership fee of this club is 10800 yen per month (86 Euro or US$ 96).

Just about the owner of this restaurant, Takafumi Horie (堀江貴文)... Takafumi Horie was a former president of LiveDoor which was online based company. January 2006 he was arrested for illegal trading practice of LiveDoor stocks at Tokyo Stock Exchange. Takafumi Horie was found guilty and after number of appeals he went to jail on 2011 and was out on 2013. Company itself is no longer after stock was delisted at Tokyo Stock Exchange on 2006.

When Takafumi Horie was a president of LiveDoor he loved media attention and appeared on numerous TV programs, radio programs, and magazines. At peak Takafumi Horie appeared on many variety shows on TV, with online business and his love of media he has become well known even among young generation. He kept no secret about his lavish life style, vacationing on his private jet with number of his female friends. It was obvious that he loved to talk about that in media.

Lately Takafumi Horie started to appear on TV programs again, when he came out of the jail TV did not take chance on him, but seems like Takafumi Horie still has love of media attention and slowly made him way to TV programs. On a TV variety talk show he appeared, he mentioned about his membership only wagyu beef restaurant he owns, Wagyu Mafia, and suddenly this restaurant became known to people. I think he just loves to talk about his life style (or image of life style he "supposedly" has) to public.

Just in case you did not know where Wagyu Mafia came from... Another memory of Bubble Era of Japan.