Write the next one down.
Five free, and the other person never pays.
For flatmates
Everyone agreed to the rota. Nobody agreed to the rota in writing. Three weeks later there is no rota, only a grievance.
Why a group chat does not do this
Everything in a house share is already agreed somewhere in a chat, buried under two hundred messages about a parcel. Scrolling back to find it is the argument, not the evidence.
“I’ll sort the wifi bill this month”
Money, without being the person who chases
Somebody covers the wifi and gets it back next month. Somebody buys the shared thing. Somebody owes the deposit share they will absolutely transfer on Friday.
Writing it down means nobody has to be the one who chases, because the reminder does it. The app asks whether it happened; you do not have to.
For the larger ones you can ask for a signature, which is worth it when a number is involved. How signing works.
Moving out
Deposits, damage, the thing that was definitely already broken. What people agreed at the time is a great deal more useful a year later than what anybody now recalls.
Five free, and the other person never pays.