This message is a bug some people are getting when trying to join the clan. We had the same issue with clan 1 and it seems to happen once the clan membership gets to the old cap of 75. But when I look at the clan members, you're already there. I know the clan tag isn't showing under your name in game and I might have found out something. When I look at your profile, it says you haven't played the game yet (which I know you have!). If Bungie.net thinks this, then it won't show the clan tag because technically you can only join a clan once you've done the intro story mission and gone to the tower for the first time. So that's possibly the "how", but I still can't figure out the "why"! Are you playing under a different PSN, maybe?