How to add a Joint Sponsor to an existing case? Adding a Sponsor to a case is easy and allows the user to assign a form to that person.

The Members tab is the place to add-edit-view-delete the Beneficiary, Joint Sponsors and Derivatives/Dependents.
Go to an existing case.
Click the Members tab
Click the + Member button → Joint Sponsor
Input the Sponsor of - the person that needs a sponsor.
Input the Sponsor - the person that is the sponsor.
Click Save.
Add Contact - If the Sponsor does not already exist as a contact then click this to quickly add a new contact to the system.