So... there's this thing called AMPTGARD that is very similiar to this LARP. A friend of mind is very very into it. There are "levels" and abilities based on how often you go to it, etc. And last I heard they played at Memorial Park in Houston or Strawberry Park in Pasadena, so this isn't some far away thing. One big from me.
It's not LARP, but it's AMPTGARD, and it's basically the same thing: http://www.amtgardatlas.com/texas.php#houstonregion Houston (Memorial Park next to the baseball field): Saturdays 2:00 PM Pasadena (Strawberry Park near the pavillion): Saturdays 2:00 PM
I know there's been a Vampire LARP the past two years at Owlcon. I know there was a Cthulu LARP two years ago. Not sure about last year. Yes, I admit it. I've LARPed before. I don't think it's that fun, but some people seem to really like it. OwlCon is a LOT of fun if you're into any sort of table top RPGs.
I just realized that LARP was not an organization per se, but an acronym for Live Action Role-Playing. Sorry to be the last one to arrive at the party.
A bunch of black and white guys formed this sports LARP in Los Angeles a few years back. We called them the Clippers.