I wont say that those pump and dump groups don't exist, and since crypto trading is so recent, and anyone can take part, we probably still see people getting scammed that way, but I think the effect of those groups are getting smaller and will soon disappear as the market liquidity increases. Big money investors don't need those groups, and they can move markets by themselves right now, and I think that a lot of the pump and dump we are seeing, is done by big whales. They move the market, create a hype around it, and then manage to slowly cash out as other join, because they thing a pumping is happening. All these things will decrease as soon as we have decent market makers in the exchanges, and it would be impossible for big whales, and pump groups to move the market.
Those groups do not target big coins like bitcoin, you need to be a whale to impact the price of bitcoin at all, what they are doing is to look for coins with a very low market cap, those that know what it is going to happen begin to buy coins without arising suspicion then they tell their followers the name of the coin they want pumped and then they cashout and they get the benefits while everyone else gets losses.