Due to file system structures, it is not possible to add a second system user to an app within ServerPilot.
However, you can use a file manager, such as FileGator, to allow another developer or a client access to your server so they can upload files to an app.
First, visit FileGator's pricing page and select the plan that works for you.
After completing your purchase, download your FileGator archive from the confirmation page.
Now, using Cyberduck or your preferred client, SFTP in to your server as the system user that runs your app.
Drag and drop the FileGator archive from your computer's Downloads folder into your app's public folder located here:
Click once on the FileGator archive and select Expand Archive from the Cyberduck File menu. Delete the FileGator archive.
Now, enter the following URL to access FileGator on your app, replacing YOUR.DOMAIN with your app's domain:
Log in with the default administrator credentials found in the README.txt file that was also created when you installed FileGator.
Change your admin password by clicking the gear on the home screen.
After securing your admin account, review the FileGator documentation to learn how to create new users and how they can upload files to this app.