To explain the way you have to enter an expense, we use an example. There is a student house with 4 room mates, Daan Bart, Fleur and Anne. Today Bart cooked and paid for the groceries. Bart, Daan and Fleur join the diner and Fleur has invited a friend that also joins. We enter this as follows:
Paid by: Bart
Date: default set to today
Amount: total amount of the groceries
Description: short description like "Groceries for diner"
Optionally you can add a picture of the receipt
Under participants we enter for each room mate their share in this expense. In this case we choose Daan 1x, Bart 1x and Fleur 2x because she's also paying for her friend. Anne's share we leave at 0x because she didn't join the diner.