Firstly, the set of urbex people overlaps greatly with the set of people who do extreme sports. So if you know anyone into parkour, rock climbing, skateboard and stuff like that, chances are, they would also really enjoy urbex, even if they've never done it before, and you could ask them if they would be interested to explore something abandoned with you
Secondly, a great way to find even more urbex friends is... meeting them at the abandoned places themselves. Abandoned places have the reputation of being really dangerous, but in reality, going into an abandoned building would be generally safer than taking a simple walk in the city, because the greatest risk and the biggest danger lies in other people.
Abandoned places barely have any people. You are very unlikely to meet anyone there, therefore it's really safe. And if you do meet someone, it's probably going to be people who are also into this rare and niche hobby. It's very easy to become friends with randoms you meet at an abandoned place, I made a bunch of friends that way.
A situation when two or even three separate companies bumped into each other somewhere abandoned, merged into one and continued the adventure together happened to me quite often. And you're likely to hang out again and become good friends with people you met that way
Thank you so much!