Splitwise works like this: you create a group for your trip, call it "Santorini Summer 2025". Every time someone pays for something, they enter it in the app, along with whom owes them what percentage of this specific expense, e.g. Ana owes 50% or 100% of this bill to Paolo, or maybe the bill is split evenly by everyone. This was all the way back in 2017, and I'm still surprised to hear how in all this years this app hasn't gotten more popular. Now I can't travel in groups without Splitwise as our personal travel accountant, it's one of the best apps for travel planning.