When will Business Partner and company level users be able to view the "+ Company" button?
Business Partner and Company level users will be able to view the "+ Company" button, once they have selected a company and navigated to the Configuration screen, on which the "+ Company" button will appear in the top right corner.
This is due to security roles linked to company groups that the user may belong to, which will dictate if the user has the relevant permissions to create a new company.
Navigate > Configuration > + Company