Most difficulties on group trips are about money, and most of those are about expectations that were never stated rather than about anybody being unreasonable.
The conversation nobody has
The one that prevents most of it.
Before booking anything, establishing roughly what each person expects to spend, and on what.
People have genuinely different budgets and genuinely different priorities. Somebody who would rather stay somewhere basic and eat well is not compatible with somebody who wants the reverse, and neither is wrong.
Having that discussion is awkward and considerably less awkward than discovering it on day three.
The question that gets the most useful answer is not what somebody can afford but what they would be comfortable spending, which people answer more honestly.
The structural decisions
Three that determine most of the friction.
Whether accommodation is split evenly or by room quality. A group taking a house where rooms differ substantially produces resentment if split evenly and awkwardness if not.
Whether shared meals are split evenly or by consumption. Even splitting is simpler and disadvantages anybody who drinks less or eats less, consistently.
And whether activities are collective or individual, which determines whether somebody who does not want to do something is expected to pay for it.
Deciding each explicitly, in advance, removes the negotiation from the moment.
The tracking problem
Practical rather than interpersonal.
Expenses accumulate across people and days, and reconstructing them at the end is unpleasant and inaccurate.
Shared expense applications handle this well, calculating who owes what and minimising the number of transfers required.
What matters more than the tool is that everybody records as they go rather than remembering afterwards, and that the currency and exchange rate used are consistent.
A shared document works as well as an application provided everybody actually uses it.
The person who pays for everything
A pattern worth avoiding deliberately.
One person putting things on a card and being reimbursed is convenient and produces an accumulating imbalance that becomes uncomfortable to raise.
It also gives that person a disproportionate say, since they are carrying the cost.
Rotating who pays, or settling at intervals rather than at the end, addresses both.
Settling every few days is the arrangement I have found works best, since the amounts stay small and nobody is carrying an uncomfortable balance.
The cancellation question
The situation that produces genuine disputes.
Somebody withdrawing from a booked trip leaves costs that were committed on their behalf.
Agreeing in advance what happens is unglamorous and it is the thing that preserves friendships.
The reasonable default is that non-refundable costs committed for somebody remain their responsibility, and that the group makes reasonable efforts to fill the place.
Stating that before booking, rather than discovering the disagreement afterwards, is the whole of the remedy.
Different financial positions
The situation that requires the most care.
Groups frequently contain people with substantially different means, and the person with less has the least ability to say so.
What helps is the organiser proposing a budget level rather than asking people to object, since objecting is harder than agreeing.
Building in optional activities rather than collective ones allows people to opt out without it being visible.
And avoiding arrangements where somebody subsidises somebody else, which is generous and produces an obligation that is difficult to carry through a week.
What I would agree before booking
A short list that has prevented every dispute I would otherwise have had.
The rough per-person budget.
How accommodation is split.
How meals are handled.
Whether activities are collective.
What happens if somebody withdraws.
And who is tracking it and how.
Six answers, one conversation, and it is considerably easier to have before anybody has paid for anything.
Booking in one name
A practical exposure worth planning around.
Where one person books and pays for accommodation or transport, that person carries the cancellation risk and the dispute risk on behalf of everybody.
If the group falls apart, they are the one holding a non-refundable booking.
Splitting bookings where possible, or collecting money before paying rather than after, addresses it.
And for anything substantial, agreeing in writing what has been paid on whose behalf, which sounds excessive among friends and is precisely the situation where memories differ later.
Deciding what to do
The non-financial version of the same problem.
Groups default to consensus, which reliably selects the least objectionable option rather than the best one.
What works better is allocating days — one person chooses the plan for a day and everybody else goes along, rotating through the group.
It produces more interesting days, distributes the planning effort, and removes the negotiation that consumes mornings.
The related principle is that splitting up for part of a trip is normal rather than a failure, and groups that permit it explicitly are considerably more relaxed than those that do not.