Count meals and people by day
A weekend away can have different headcounts at each meal. Write “Friday dinner five, Saturday breakfast seven, Sunday breakfast four” before dividing the shopping. Ask who arrives after dinner and who leaves early. A single guest total can easily overstate the groceries for the first and last meals.
Assign the items that must meet in one place
Check what the accommodation provides. A kitchen with no large pan can change the dinner plan more than a missing condiment. Keep items already packed separate from promises to bring them, and ask each person to report an omission while there is still time to buy it locally.
For pancakes, having flour in one car and the only pan in another works only if both cars arrive before breakfast. Put the main ingredients and essential equipment with an early arrival, or choose a meal that does not depend on that pan. Mark confirmed items as packed rather than merely volunteered.
Choose what happens to the extras
Before departure, identify which ordinary items the host already has and which leftovers someone can take home. Ask about household preferences or requirements directly instead of inferring them from the group's previous trips. If plans change at the last minute, update the affected meal counts and notify the people buying those ingredients.