Is the questioner sinful for not denouncing the female students who post immodest pictures in a mixed study group on Telegram, given his knowledge of the transgression and his attempt to warn against the excesses?

It is permissible to join university groups for the purpose of personal benefit and benefiting others, while avoiding reprehensible acts and enjoining against them as much as possible, in accordance with the saying of the Messenger of Allah, peace and blessings be upon him: "Whoever among you sees an evil, let him change it with his hand; if he is not able, then with his tongue; if he is not able, then with his heart—and that is the weakest of faith." It may become incumbent upon a person to change an evil if no one else is aware of it or is able to remove it.