How Do I Fix an Error on Copyright Application

Dear Rich: How do I correct a mistake on a song copyright that I recently filled out? I need to add a third songwriter. 
We assume you're referring to an error made on a copyright application you filed online. If your application is still pending (that is, the Copyright Office hasn't issued a registration certificate), you can submit a request to address the error or omission on the Copyright Office’s website. Alternatively, once the application is being reviewed, you can inform the registration specialist (the Copyright Office examiner of your application) of the required changes.
Registration issued. If the certificate was issued, you should file a supplementary registration to correct errors or omissions. This is done online using the ECO portal. To learn more about supplementary registrations (and the exceptions to online filing), read Circular 8.