Hey there, and welcome to Apple Support Communities, laird80!
From our understanding, you have questions regarding Numbers, and collaborating with others. We’d be happy to help out!
You mentioned that one person is unable to access the shared spreadsheet, we’d recommend that they follow the steps listed under “If collaboration isn't available” at: About collaboration for Pages, Numbers, and Keynote - Apple Support
In reference to your second question, Numbers does not have the capability to set access to particular cells within the document. However, we’d recommend that you provide feedback regarding that at: Product Feedback - Apple
Cheers!