Look at Scientology and lots of other cults like it. The people in it aren't stupid. Brainwashed, perhaps. There are many tricks both seemingly logical/cognitive and emotional that people can be taken in by. The emotional side is important: these people show affection to people who join them, and they often target people who are at uncertain points in their lives. Cult-style groups have often gone after college students precisely because they are at a point in their life where they are unsure about their future and potentially emotionally volatile.
There's a lot of people out there who are living unhappy lives, who find themselves alienated from society, who have a strong but unfulfilled desire to join together in a community with others, who lack self-confidence or enough self-belief to push back.
Cults aren't just stupid people making stupid decisions: sometimes joining a cult is a decision made by an intelligent, educated person in a moment of weakness. And the way the thing is set up makes it not seem cultish when they are joining. Scientology don't tell you about Lord Xenu on day one. They lure you in with promises of making you a better person: reducing your stress and anxiety, helping you with addictions to drugs, tobacco, alcohol etc., helping you succeed in business or education or whatever, helping you have a better relationship with your partner or family. Out of that menu, there's something everyone will want. Those are all the same things advertisers lure people in with. Falling for that kind of pitch doesn't make you stupid, it makes you human.