The Hollow Hills Society is a society meant to foster good will between humans who know about the supernatural, and value their continued secrecy, and supernatural or otherworldly creatures. In the larger world, it presents itself as a cryptid, UFO and conspiracy enthusiast club and support group. It holds various fundraisers (for research and support) and events (just for fun! and for information, support, etc.) among the general population. Those who "believe" can pick up the clues about its true purpose, and perhaps pick out the supernatural members of the group, who will then decide whether they can be trusted with a higher-tier membership.
The Hollow Hills Society has existed for hundreds of years. Its modern, public incarnation is much younger, having existed for about 20 years.