Your login credentials only allow you to access one account when created, regardless of whether it was generated as part of creating a new account or by adding someone.
Depending on the situation, the login can be given the ability to login to multiple accounts.
Before you start
Things you need to know:
If this is the first time you want your account’s login to access multiple accounts then you will need to contact support to arrange this.
Grant multi-account access
If your already have a login with access to all of the accounts and you now wish to give the same access to other logins, you can:
With the login that already has access, login to the account where the other user exists, or create a new user in one of the existing accounts.
Expand the User menu and go to Settings > Account Details > User Accounts.
Find the user you want to grant the permission to and select ACCESS PERMISSIONS.
Select the Client Accounts tab at the top of the page.
Select the check-boxes next to each of the accounts you want to give them access to.
Select UPDATE ACCOUNT at the bottom of the page.
Now when the user logs in they are presented with the list of accounts they can access.