I think it depends on how you want to spilt the costs and what everyone is happy with.
Are the people staying the week happy to pay more? If not then the people only coming for 3 days should still pay the full amount.
If it’s split equally over the week regardless of how many are staying it’s £51.67 per person per night or £258.35 total per night. So £310 per person for the week.
it gets complicated if they are only paying for when they are staying
It would still be £258.35 per night
For those staying 3 nights, they would pay £155.01 each for the 3 nights.
The other three nights left would then be split between the 2 that are staying having to pay extra.
eg
nights 1-3 5 people staying would be £51.67 each totalling £258.35 per night.
nights 4-6 2 people staying £129.18 per night again totalling £258.35
So those staying 3 nights would pay a total of £155.01 and those staying the full week would have to pay £542.55
Hope that makes sense!
Its a huge difference for the extra few nights.
If they originally agreed to go for the week then I think it’s only fair they pay for the full week regardless of how long they stay for.