Originally, didn't the developers compare dragons to dolphins in intelligence? But more recently I remember there being a tweet or a post or something in an interview (sorry so vague) saying that dragons were as smart as a smart dog. So, basically, border collie level intelligence is what I expect; they've been toned down from their original state
Dolphins are ridiculously smart, although some of their intelligence is alien to us--they are more emotionally smart vs. logically smart and I remember reading somewhere that they may even experience emotions that we humans aren't capable of feeling. So, if dragons are dolphin-smart, then they'd probably have more intelligence than us in some area other than we expect--I doubt social intelligence, but then again they do live in groups of high dragon+drakes+dragonlings.
But dogs... well, dogs aren't stupid. They don't compare to humans like dolphins do, but they can definitely figure things out and sometimes they can even surprise you. And, if dragons are as smart as dogs, well... dogs (and yes, also wolves) can certainly work with people as the cultists did, especially when they receive some reward or benefit from it.
Unless, however, someone intends to tell me that once upon a time long ago, dragons were domesticated and, for instance, used as war-beasts by their owners (which, okay, could almost work considering the dragon cults and the Old Gods etc. but not really, I think), I'd say it's less that they are dog-level intelligence and more that they are wild-canine intelligent. Dogs, African painted dogs, foxes, etc. All of them are pretty intelligent and most of them are social--which makes them a better comparison than most cats.
Instead of dogs, though, I would argue dragons in the DA-verse compare better to hyenas. Hyenas are led by females, who are larger than the males. They actually hunt more than they scavenge, despite their reputation as scavengers--lions scavenge more often than hyenas do. They are social animals, and while I don't know precisely how smart they are, they are considered very intelligent by experts who study them. They can also be trained, apparently with surprising ease considering they're wild animals with no history of domestication. That bit, in particular, lends them to compare well with DA-verse dragons (versus wolves, who can be trained but don't always pick up on things easily/resist training as compared to dogs).