So, my very first boyfriend was a LARPer. I had zero interest at the time, but might actually be talked into it now. Funny how life works.
Anyway, so he heard about it on our college campus. There was a flyer up in the dorm or our commons; probably in our campus daily newsletter too. They may have been an official campus club, but i can’t tell you for certain. They were open to community members joining as well.
I don’t think they ever did any conventions; they did meet up occasionally with another LARP group from a nearby city. I believe there was some kind of an internet community where they were able to find and contact each other. The ex actually became a leader of the other group when he moved and got more involved with them. I would say that kind of community, much like any other hobby, has their web forums, sites, facebook groups, etc, to find and communicate with each other.
In my experience as a Con-goer however, most LARP (or LARP like) events have a sign-up form and are officially sanctioned as part of the convention. That means, they end up in the programing book and are under the jurisdiction of Programming. Anyone who wants to participate can contact them for more information, or be directed to the leads of the event by them. I will say, MOST cons have things like “Boffing Tournaments”; more than an actual LARP. At the end of the tournament, there is a single winner and they receive a large prize (like a real sword and a “belt” for example). 2nd and 3rd place people might get smaller or replica pieces, plastic trophies, or ribbons.