Same share or by headcount
Divide evenly per household, or by how many people each brought.
There are two ways to divide the total. They only produce different numbers when households are different sizes. On an event where everyone comes alone, both give the same result.
Everyone pays the same
One share per invitation, no matter how many people that invitation brought.
By headcount
Shares are proportional to people. Adults, children and grandparents each count as one person.
The same event, both ways
Everyone pays the same
$120 divided between three households.
By headcount
The same $120 divided between the eight people those households brought.
Which to choose
Match it to what the money went on. Headcount suits spending that scales with people, such as food and drink. The same share each suits a fixed cost the group used together, such as hiring a hall, decorations or a booking fee, where the number of people made no difference to the price.
Note — Under headcount a child counts as a whole person. To charge a particular household less than their headcount implies, set a fixed amount for them rather than changing the method for everyone.
Events where everyone is invited individually offer only everyone pays the same, since each household is one person and headcount would produce identical figures.