As an administrator, you can set the default company for a user or another administrator. An administrator can only use the companies that are available on their own profile and have access to.
Set a User's Default Company
From the Settings menu
Select the Users button
Open the specific user
Select the default company
Set a User's Available Companies
While on the user detail screen
Add additional companies in the available companies area
User will only have access to these companies
As an administrator, you do not have access to all companies by default. This is so that the system can control which administrators have access to which companies and additional functionality, such as creating and deleting companies.
Click here to understand how the global company list permission works.