From https://www.hplhs.org/about.php

The HPLHS officially came into being around 1986 in Boulder, Colorado. Prior to that, we were a bunch of friends who met doing high school theatre in the early '80s. Sean Branney had the classic Chaosium role playing game Call of Cthulhu, and invited Andrew Leman, Darrell Tutchton and some other friends over to play it. Those frightening and thrilling nights when we gathered around a kitchen table in Englewood, Colorado paved the way for many strange and improbable events that followed.

While we all enjoyed playing an RPG game around the table, as theatre folks we were driven to, well, act. We thought the game would be even more fun if we put on some costumes, hid some clues around town and played the game in character out in the real world. At this point, we’d never heard of the idea of Live Action Role Playing (LARP) games, so we invented a system of rules that served our purposes called Cthulhu Lives!™ Soon we were staging games in creepy basements, vintage mansions and deserted mountainsides in Colorado. Many friends played in these games with us, and for most of them, once they tried it they were hooked. In one notable gaming incident, Sean ran an adventure on the campus of the University of Colorado at Boulder in which black-clad ninjas attacked some of the investigators on the college campus. Passers-by who did not understand what was going on alerted campus police, and when they demanded an explanation from a quick-thinking Sean, he told them the group was the H. P. Lovecraft Historical Society. The police were mollified, and the name stuck.

In a Boulder pizzeria in 1986, Sean Branney, Phil Bell and Andrew Leman decided to make the group a real official thing, and The HPLHS was born. People in other parts of the world heard about what we were doing and were interested. We published a small monthly fanzine called Strange Eons which documented the group’s activities and published articles on Lovecraft, history, and other items of interest. Even in the pre-internet age, we soon racked up nearly 200 members across the English-speaking world. As a college project in 1988, the HPLHS produced its first motion picture, an adaptation of "The Statement of Randolph Carter" that was shot and edited on VHS videotape.