Negative mitzvah #10 = “לא יקום עד אחד באיש לכל עון ולכל חטאת” This is a prohibition against testifying by yourself. Normally a court only accepts two or more witnesses as testimony. There are special circumstances like prompting a defendant to take an oath where a single witness is accepted. However if there is no use for the single witness it is forbidden for him to testify in court and is considered Loshon Hara.
Negative mitzvah #11 = “לא תהיה אחרי רבים לרעות” You can hang around a bad crown, like a group that speaks loshon hara. The Be’er Mayim Chaim points out that not only is there a Torah mitzvah against hanging around bad crowds but in the rest of Tanach there is many instances where it mentions the same thing so it is also Rabinnically forbidden, for example the first verse in Tehillim, “The praises of a man are that he did not follow the counsel of the wicked, neither did he stand in the way of sinners nor sit in the company of scorners.”
Negative mitzvah #12 = .”לא יהיה כקרח וכעדתו” There is a specific prohibition to not act like Korach and his followers. Speaking or even accepting loshon hara can lead to intense fighting. If the person listening would have made a displeasurable face, showing he does not want to listen to this type of talk, there is a good chance an argument could have died down.
Negative mitzvah #13 = “לא תונו איש את עמיתו” This a prohibition against speaking אונאת דברים, speech that makes people feel bad, in front of them bythemselves is a problem, all the more so if there is a group listening. Examples are talking about what a baal teshuva use to do before he or she became Torah observant, disabilities in intellect, physically, emotionally, etc., or even talking about family members in a negative way. If there is no helpful purpose it is forbidden to bring up even in private, all the more so in public.