When you want to allow an external user to upload assets, follow these steps to get the link allowing them access.
Share External Upload Link
Go to the Assets tab.
Scroll until you see Sets.
Click External Uploads.
Copy the link at the top of the page.
Paste the link into an email to the external uploader.
Tip: If you click the link while logged in, it will take you to the normal upload set options. To see what it looks like to external uploaders, log out first and then click the link.