Emphasis on the prohibition of  Lashon HaRa (slander) is amazingly great. I only wish I had been more careful about this myself. In fact, everyone that I ever knew that had success in Torah learning were always extremely careful about this. In fact, I would say that success in understanding and keeping Torah always seemed to depend on carefulness in Lashon Hara, and not at all in intellect. 

I saw very smart people that did not get very far in learning, and simultaneously they also were not careful about Lashon Hara. 

I also saw people not so smart, but that learned and understood Shas [Talmud] very well, and it always turned out those were the guys that were careful about Lashon HaRa. 

But for myself I should mention that warning people about bad groups in not in the category of Lashon Hara. Still I wish I had spend more time on the Chafetz Chaim. 
Maybe things would have been better if I had. 

For the general public let me just give some background. Slander has its own verse in the Torah. But there are plenty of other verses that are applicable to it. The actual verse is לא תלך רכיל בעמיך "Thou shalt not walk around as a tale bearer among your people." Leviticus.