Ponzi schemes often appear promising and look to yield enough profits when you engage in them and this is what lures most people. Personally, one easy way to detect that it's a scam is how they never tell you the downsides, always putting the positives in front. You did the right thing by leaving them be because interfering in the situation will look like you're being a killjoy or you are not profit oriented.
Most of these Ponzi schemes thrive on telegram channels and I still get amazed how people fall so foolishly for those whack tricks.
 But if I had my way as the op, I'd at least tell them the danger of what they're doing so that even if they don't listen, at least I had a clear conscience and so that if it doesn't pan out well, I could either gloat or it could be a good platform for preaching Bitcoin to them. At least I'd be transparent with them on the ups and downs of investing in Bitcoin, which is something these Ponzi schemes lack.