How can I acces and assits build a friends form?

If you need to access a form from another account, you will then need to request the Owner of the form to share their form with you as a sub-user as shown in the guide below.

https://www.jotform.com/help/232-How-to-Share-Forms-with-a-Sub-Account-User

You can also request user  to add you as a collaborator to their form. With this you can also make changes to the form.

https://www.jotform.com/help/419-Understanding-Form-Collaboration

Kindly note that sharing form is a process that will be initiated by the form owner and not Jotform.