Well, people are barely "joining" it today. It's mostly some of the children of members joining because of pressure from family and growing up in it and not knowing better.
But since there are still some adult recruits joining, and since I was an adult when I joined, I will try to answer. Granted, my joining was preceded by childhood exposure to the beliefs as the only religious training I received, but I can still try to answer.
I joined because I was in a state of emotional turmoil. I needed some kind of comfort, and the JW's had a false offer of that comfort. Some would join for answers to their problems, be they substance abuse, loneliness, boredom, some may join because of what JW's seem to have- instant answers to everything with their paradise and destruction of the wicked.