User 'A' has Full Control on the document library. User 'A' also has full control for the specific Excel document.
User clicks on 'Manage Permissions' for that Excel file, clicks 'Add New User' and receives an error message 'Error: Access Denied'
I've checked top level site settings and worked my way through each folder and User 'A' definitely has Full Control.
The user can edit someones permissions, but they can't add a new user.