The rate you looked up is not the rate you paid
Four people take a trip to Norway. Someone pays for the rental car in kroner. Back home they open a currency app, get today's rate, convert, and enter a dollar amount. It is close. It is also wrong, in a way that compounds across a week of expenses until the balances no longer match what anyone's card was charged.
Three rates are in play on any foreign charge. The mid-market rate you see quoted. The network rate that Visa or Mastercard applied on the day the transaction settled, which is not the day you spent it. And a foreign transaction fee, usually up to 3 percent, unless the card does not charge one.
Only the second and third leave a mark on the statement. That mark is the number the group owes against.
Pick one rule and hold it for the whole trip
Any of these works. Mixing them does not.
Settle in the local currency. Record every expense in kroner, split in kroner, and convert only the single final balance each person owes. One conversion, one rate, no drift. It is the cleanest option and it needs everyone to be comfortable seeing balances in a currency they do not think in.
Use the posted charge. Whatever the payer's statement says the charge cost them in their home currency is what the group splits. This is exact, because it is not a conversion at all. It requires waiting a day or two for the charge to post, and it means the person with the worse card carries their own fee unless the group agrees otherwise.
Fix one rate for the trip. Agree on a rate on day one and apply it to everything. Slightly wrong, consistently, which is much easier to reconcile than each expense being slightly wrong in a different direction.
Who pays the foreign transaction fee
Worth deciding before the trip, in one sentence, rather than discovering afterwards that one person absorbed 3 percent of every group dinner.
The common answer is that the payer eats their own card's fee, because they chose the card. The fairer answer, when someone volunteered to put the whole rental car on their name, is that the fee is a trip cost and gets split with everything else. Either is fine. Silence is not.
What this looks like in Footnote
A charge in kroner arrives from the bank already carrying its original amount, its converted amount, and the rate that was applied to it. The split runs on that, so nobody types a rate in and nobody has to trust that the person who did looked it up on the right day.
Open a balance afterwards and each line shows the foreign amount, the rate, and the share. That is the difference between a group agreeing on a number and a group being able to check one.