“Everyone was doing it” is a headcount, not a defense.

Counting how many people did something describes the weather rather than defending anything. The number is often perfectly true, and it answers a completely different question from the one being asked, which is whether the thing was all right to do. A crowd can make something feel normal, and normal and acceptable come apart regularly.

Where you catch yourself reaching for the crowd, finish the sentence properly: it was fine because, and then say why. If no reason turns up, you have your answer before anybody else hands it to you. Run the same test when somebody offers you the crowd as a reason.