So the denizens recruit mutants, the corrupted, and the damned into their ranks. The simplest method is through cults. Zones of weirdness, evil satellites, strange stars, monsters, even the howling gods themselves, they all send little Shaver Mystery-style signals into the brains of the susceptible.
Humans are also unfortunately prone to depravity on their own. Even those who start out virtuous may become monsters after seeing enough death or hardship. It's so much easier to just give in. You'll be rewarded for it, in fact. You'll come to see that it will help people, in the end, if you follow the twisted logic. Just give in.
Typo hunting appreciated.