Hafetz Haim Halachot
Halacha 12 / 215

Hilchos Lashon Hara, Opening – Negative commands (4)

Both the speaker and the listener also break the negative commandment (Vayikra 19:14), "Do not put a stumbling block before the blind." Each one puts a stumbling block in front of the other by causing him to break explicit Torah prohibitions.

But here there is a difference between the speaker and the listener. The speaker breaks this commandment whether there are few listeners or many. In fact, the more listeners there are, the worse it is.

The listener is different. He might break this commandment only if he is the only listener, because if he did not listen, the speaker would have no one to tell the lashon hara to. But if there are other listeners, perhaps he does not break this commandment - only the other negative commandments listed in this Opening. All this is only if he joined after the story had already started. If he was there from the beginning, he surely breaks it in any case, because the prohibition started because of him.

Either way, a person must stay away from such groups and not sit with them, because in Heaven they are all written down as "a wicked group." This is what Rabbi Eliezer the Great commanded his son Hurkenus in his will: "My son, do not sit with groups that say bad of their fellows, because when the words rise above, they are written in a book, and all those standing there are called by the name of a wicked group."