Hello Matt,
I understand that you want to share an Excel spreadsheet at Office 365 and use password to control accessibility.
An Excel spreadsheet can be viewed or edited at bowser via Excel Web App directly. However, setting a password for Excel file at SharePoint Online is not available currently.
Moreover, we can use other tools, such as permission control, document library and custom lists, to achieve the requirement via SharePoint Online service.
As a result, please provide detailed information about the requirement, such as whether the users are internal or external, the architecture of the Excel spreadsheet and a detailed example of the requirement, to find a solution.
Thanks,
Claud