The problem with this type of "investment" is that almost every time operator runs away with all money and stole from everyone who invested in.
My thought are: if you don't know whether it is a ponzi or not because operator disguised it as some standard investment it is definitely a crime.
But if you knew exactly that this is ponzi and operator stated that plainly, then it is ok to run these type of activities. Some people would like to gamble and earn big money this way.
They know if they are late they are losing everything and if they pull out in time they will earn.


All in all. It is a grey area and I would never recommend investing in known ponzi because it is too risky.