“Everybody was doing it” has never once split the bill.

Groups provide cover in the moment and absolutely none afterwards. The people who were also there do not appear when the consequence lands, and there is no mechanism by which shared participation becomes shared cost. The comfort of numbers is entirely a feeling. It has no standing anywhere that matters.

In a moment where everyone is going along, quietly ask who will be with you tomorrow if this goes wrong. Usually the honest answer is nobody. That question is faster than an argument, and you do not have to say it out loud.