Find the next decision deadline
Read the room, transport and ticket confirmations separately. One booking may allow a change until noon while another has no change option. Ask the provider what can still be refunded, transferred or reduced for the actual group size. Record confirmed answers rather than assuming all parts of a trip follow one policy.
Revisit the original group agreement
Suppose four friends reserved a $400 room and one cannot travel. If the room price stays $400, the change in headcount alone does not tell you who should cover the fourth share. Look at what the group agreed before booking, then discuss what is fair in light of the new circumstances. Do not collect new payments before that discussion.
Name who will act and what counts as done
Make the decision before the next relevant deadline and write down who will contact each provider. If there is no refund, revisit the agreement everyone accepted when booking. If it left cancellation costs open, discuss the remaining amount without assuming a particular person must absorb it.
One adult can contact the host and another can check transport, but each needs a deadline and an expected result. Save confirmations of cancellations and any money actually returned. A provider agreeing to review a request is different from granting a refund. Keep open amounts visible until the people contributing know the outcome.