No free lunch theorem. There is no free lunch, remember that and you will figure out facts behind free signal groups.
They do not charge you any joining fee initially but they will steal your money by giving you some signals and make you excited, greedy to buy up tokens for them to take profit.
Paid groups can be scam too. They can be other versions of free signal groups but try to double scam you with joining fee. Very few paid signal groups are legit and good but you must pay ridiculous expensive joining fee. I am sure you will not be able to join those groups if you are not rich and have big capital.
You can use your money to buy a membership from on-chain metrics websites like Glassnode
https://studio.glassnode.com/pricingGlassnode Insights has weekly free reports to read
https://insights.glassnode.com/ <- What he said. Everything you need to know.
If you like the thrill of "investing at the right time" you can also support your local fortune teller instead.
I am talking about Bitcoin signals. Unless the group is connected with mega-whales (very very very unlikely), they will not have an effect on the market price.
If they are giving signals for shitcoins/tokens, yes, they might "predict" some mega jumps/pumps correctly. The reason being, these markets can be easily manipulated with small amounts of funds. That does not mean it will be a profitable endeavour for you, though. The only ones profiting from the pump will be the owners of such a group, the signal makers.
Just stay away from this type of stupid scam-game and HODL.