As part of management settings, admins have the ability to move users from one sub-company to another. To do that, simply delete the user(s) in the original sub-company, and then create a new user with the same name and email within in the new sub-company.
Please note that when creating a user in the new sub-company, the access credentials will be resent, in addition to a welcome email that will include a new default password.
To learn more about visibility permissions, please read What can users view on inspections?