Apparently the cult stuff starts with a text from Ron, but I just must've glossed over or something 'cause I don't remember anything about it. I found where the cult is, but I don't know how to start any missions. Do I just bring some hitchhikers to the camp and that's that? Also, is there a finite number of hitchhikers, because I've already picked up a few and taken them where they wanted to go?

When you pick up someone as Trevor the option pops up on your map as an "A". The dialogue is pretty great.

There is also some cult stuff with Michael if you go to the epsilon website.

I finished the game without ever doing any cult stuff, tried to pick up a woman crashed on the side of the road and they had the dialogue about the cult but when I got there (yes as Trevor) nothing happened, I was just parked outside of the gate.

edit: it wasn't marked on the map or anything, I had to use a guide.

Thats because that woman becomes part of your crew for heist, she is great with near full stats and only ask's for 5% share. So the game doesn't want you to be an idiot and sacrifice her.

Any other hitchhikers are sacrificial.

As said before it is an A on the map that appears when you have a hitchhiker as Trevor. I did it and got some small money for it, I dont think its worth it.

Well I picked her up after completing the entire game, so. I wanted her dead.

I highly recommend dropping off a few hitchhikers to the cultists. There's bonus story content if you do it enough times. :D