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:

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