Events table
| Event ID | Event name |
|---|---|
| E001 | Event 1 |
| E002 | Event 2 |
Users table
| User ID | User name |
|---|---|
| U001 | John Doe |
| U002 | Jane Doe |
Expenses table
| Event ID | Expense ID | Expense name |
|---|---|---|
| E001 | X001 | Buying BBQ |
| E001 | X002 | Buying wine |
Expenses payments table
| Expense ID | User ID | Amount |
|---|---|---|
| X001 | U001 | $5.00 |
| X001 | U002 | $10.00 |
I think you should have a form to add an event, and leave the expenses and payments later.
In the details view of an event, have a collection/form to add an expense, and in an expense, have a form to add payments related to that expense. You can bring over IDs for relations inside the form using special values.