I've no idea about private groups. But, There is nothing special in the trading signals group which are so-called premium /VIP, their only aim is to collect money from people.
However, I googled to know what is the opinion of people about this.
I agree that private ones that either charge, or at least go private just to make it look like they are really good, will end up being as terrible as the public ones. If they knew how to trade so well, I can guarantee you that they would not charge you on how to trade. They would just trade and make money, why would they need your money?
It is similar to trading bots as well, if a trading bot works so well, why would someone sell it? Just put in 1000 dollars into it, and let it turn that into millions of dollars overtime. The logic never stands with these, because you can't make a profit all the time with them, the risk of losing money will always be there.