The articles describes the steps to resolve the following issue:

  1. You try to create a new account contact and get the following warning in red:
    These credentials are already registered for another account(s):
    Already registered
  2. You select the Add contact used on another account option and get another warning:
    Login is not registered for any account:
    Not registered

This means that the account contact you're trying to add already exists in Intermedia database and the password you're using doesn't match. If you don't know the correct password, to resolve the issue:

  1. Navigate to the account login page and click the Forgot password? link:
    Forgot password
  2. Authorization code will be sent to you:
    Authorization code

Use the restored password on the Add contact used on another account page.