Ahh, yeah, it's a pyramid scheme - anyone who doesn't know what this is, Wikipedia has an informative article on them. ACN also has a website.
About 5 years ago when the economy was bad (but not as bad as now) I was invited to one of their seminars. I was a sophomore in college. I think the seminar was $10-15 entrance fee. At the seminar they get a bunch of the top guys (who've apparently made a lot of money) to get up and talk you into doing it - these guys are no more than douchebag car salesmen and most of them haven't even gone to college (my take on it). They latch onto the doctors/MBAs/people with high education and really try to get them involved because it builds trust with other people they try to swindle in - look, they got a Harvard MBA to get up and speak to people, it makes them look good!
Right after the $10-15 seminar, they hold another seminar, but this one costs $500 - with promises of getting you started and netting you thousands of dollars within weeks. I remember thinking about it and then leaving. Something didn't seem right. They tell you that if you bring in 2-3 customers (I can't remember the exact amount) within a certain time period, that they'll refund the $500 fee. A couple of my buddies tried it out but didn't ever get the customers and thus lost the $500.