How to add collaborators and enable permissions to access the uploaded files?

You can add them as collaborators in Google Forms and then click on the Customize this form option in the Formfacade addon menu to sync the changes with Formfacade. New collaborators will be able to access the uploaded files.

Who can access the uploaded files?

By default, the user who installs the Formfacade addon to embed the form on their webpage or uses the SHARE link to collect responses will have access to view and download the uploaded files.

When a user uploads a file and submits the form, the url of the uploaded file is recorded as a response for that short answer question in Google Forms. You can access the uploaded files using that link.

How to add collaborators and give permissions to view uploaded files?

Open your Google Forms > Click on the More ⋮ icon > Select Add collaborators > Enter the email addresses > Click on the SEND button to add them as collaborators.
Whenever you remove or add new collaborators in Google Forms, you must click on the Customize this form option in the Formfacade addon menu to sync these changes with Formfacade.
The collaborators in Google Forms will be able to view and download the uploaded files using the link.