I know many women who joined as adults who were victims of physical abuse or some sort of sexual assault. I know many men who join because their wives do.
I think generally to join the dubs as an adult, you have to be looking for some sort of security. The world probably hasn't been very nice to you. You crave a certainty that you are protected by God, and worshipping Him the right way, that you are in with the "inside scoop". You don't have to trust your own conscience anymore because so much of that work is done for you. You don't have to figure out complex theological issues, because those are explained to you. The entire universe is tied up in a nice neat bow, and all of your activities are planned for you. You are given order, ritual, and a sense of purpose.
Any other things that make it appealing?
What sort of experiences do people have that make them more likely to join as an adult?
Molested as a child? Abused physically as a child? A chaotic home life? Has anyone with a healthy upbringing ever joined as an adult?