All applicants over the age of 18 are required to submit an application and pay the $29 fee. When you apply, you can add your co-applicants in the Your household section. Your co-applicant will receive an email inviting them to apply and they will need to submit their application through their own account.
If you have already submitted an application, and need to add a co-applicant, you will need to withdraw your application from your Renter Hub. This will allow you to edit the Your household section on the application to include your co-applicant's email address and re-submit your application.
For more information, you can refer to this article:: How do I edit my application?