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.
That's the main reason why there are private groups in Telegram that offers such trading signals. There are lots honestly.
Yes, they are giving and providing signals but obviously, only a few might be correct. Aside from that, those signals don't have an involved analysis.
Free money for these owners. If they predicted it right, it will result in their subscribers to believed in them. That's the reason why they can lure other people to subscribe and pay money for that kind of service.