Is it possible to directly list the users of the organization in a field to associate their names with my bills, instead of creating a multi-dropdown custom field?

Yes, you can create a custom field using the Lookup data type. By doing so, the users available in the organization will be listed in the custom field. To create a custom field:

1. Click the Settings icon at the top right corner.
2. Go to Purchases and choose Bills.
3. Select the Field Customization tab.
4. Click + New Custom Field at the top right corner of the page.
5. Enter the details and choose the data type as Lookup.
6. Select the module as Users, enter the related list name, and save the custom field.