All applicants over the age of 18 are required to submit and pay for their own application, under their own Zillow account. When you invite an applicant, they are able to add any co-applicants in the Your household section of their application. All co-applicants will receive an email inviting them to apply and they will need to submit their application through their own Zillow account.
If your applicant has already submitted an application and needs to add a co-applicant, they will need to withdraw their application from their Renter Hub. This will allow the applicant to edit the Your household section on the application to include the co-applicant's email address and re-submit the application.
For more information, your applicant can refer to this article:: How do I edit my application?