Do you live near mountains? Join a mountaineering club and take their classes. I was an instructor for years and it became a sort of a joke that the intro courses were transplants looking to make friends.

You’ll meet people involved in a wide variety of mountain sports. The bulk is climbers, skiers, and mountain bikers but a lot of unlikely overlap in paragliders/BASE jumpers (want to hike further and higher) and kite surfers for some reason.