Click on the Settings button on the side bar, go to the Linked Accounts page and click on Link Account.

Then, enter the username and password for the administrative user on the account that you wish to link, and click "Link Account".

The administrative user information on the linked account will then be overwritten with the administrative user's information from the master account. Company and other account information will remain unchanged. The account will instantly be linked and will appear in the Linked Accounts section of the master account.