On a trip, money gets messy fast: one person books the hotel, someone else fronts the rental car, a third keeps buying the groceries and coffees. By day three nobody remembers who paid for what. The fair outcome isn't "everyone pays an equal total" — it's that everyone ends up having spent the same on shared costs, with a few transfers at the end to even it out.
Someone joined late? Only tick them into the expenses from the day they arrived. One room was pricier? Add it split only between the people who slept in it. A group dinner where two people didn't drink? Add the food as one shared expense and the bar tab as a second expense split only among the drinkers.