Creating an agency can help you share access to already added accounts. To make sure invitations from the agency can give access to specific accounts you should make sure the company of that account is added under the agency. You can have multiple companies under the same agency but one company can be only under one agency.
Here are the steps you should follow to connect the company to the agency:
Go to the company tab by clicking on your profile icon in the top right and select Companies.
Find the company of your choice and click on the settings button in the Actions column next to your company.
Select your agency from the first dropdown menu.
Please note that if you have not connected a company to your agency any invitations sent by it will not result in that person being able to manage any account!