How do extra party members affect fullness when ordering food?

Solution 1:

Each party member adds 1,000 calories to the fullness bar.

A team with 3 party members will be full after 3,000 calories, 4 after 4,000 calories, and so on. To figure this out, I ate food before and after gaining a party member, then calculated how many calories it took to reach 100% fullness. So with a party of 4, if a burger with 600 calories fills 15%, then:

(600 cal/15%)/4 = (40 cal/1%)/4 = (10 cal/1%) = (1000 cal/100%)

As Chronocidal points out, there is an exception to this. During Week 2 Day 3, you get an option to split the party. If you agree to split the party, you will retain the full set of calories, even though only half the party is present.