I've had two brushes with people involved in these groups. The first was a friend who got sucked into Amway, and tried to get us all to join, and the second was one of those fake "job interviews" that others have mentioned. I quickly figured out that Amway wasn't worth the time or money from talking to people (this was back in 1996 before blogs were really a thing), and the second was for CutCo, fortunately, my mom and dad had given me the sage advice of "if someone wants you to pay money for a job it's a scam" so I as soon as I realized what it was, I left along with several other people who were angry their time had been wasted.
EDIT: I just remembered another one. This one occurred in January of this year. It was for some coffee thing. I looked it up, and you have to maintain an inventory, at your expense. That's a big glass of "fuck no".