The easiest place to start understanding a spectral concept, such as cults, is to indentify it's lowest and highest points rather than starting somewhere in the middle.
So lets call the lowest point of the cult spectrum benign and the highest point destructive.
A good example of an extremely benign cult would be the following of the not so popular 80s movie: The Princess Bride of which I consider myself a member (no documentation ). I was also a member of a destructive cult, as we all know, the Watchtower.
If you were to come up to me and claim to be part of the Princess Bride's cult following I could figure out whether you are a true member by asking you a few questions. Based on your answers I could then conclude that you either meet the criteria or that you do not. I might start off with some easy ones like what does ROUS stand for? And end with something difficult like "finish this scene from this line: I'm not a witch I'm your wife!" Of course if cannot pass my stupid test I am only disappointed of the fact that I can't banter with you about the movie as all cult members like to talk about the cult's content. I do not hold any different opinion about you since being obsessed with a movie isn't necessarily a good thing. I don't need you to like the moive for us to have a relationship.
On the other hand when I was in the Watchtower and you came up to me claimed to be "one of Jehovah's Witnesses" I could, once again, then ask you some questions for verification. If I didn't conclude that you were one of Jehovah's Witnesses then I would then have to immediately suspect foul play, or just think lowly of you, since you can't just say that you're one of Jehovah's Witnesses. You must first have been groomed for many months and study our literature with one of our represenatives. This represenative much vouch for you. After partaking actively in our world wide ministry as well as showing a keen interest in our way of life you could then be eligible for a special oral question and answer test. If you pass this test you can then get baptised in the True Religion, not now, but at the next convention or assembly held triannually all over the world.
^ Both scanarios involve people's preferences of being around like minded people, but one is totally harmless and one yields unspeakable horrors.
-Sab