Is enjoining good and forbidding evil always to be done in private, such that I am not permitted to enjoin good or forbid evil publicly if I find a group committing an evil, such as drinking alcohol or not wearing the hijab?

The fundamental principle for one who becomes aware of a wrong is to condemn the doer and conceal his fault, rather than exposing him. However, if the person is openly committing the sin, then it is permissible to expose him and reveal his affair so that he and others may be deterred. Whoever finds people doing something forbidden must prohibit them and change their wrong to the best of his ability. If individual advice is possible, it is better than advising in a group.