If someone - without user account - is to be given upload rights to a specific folder, a direct-upload-link can be created for this purpose.
First, navigate to the folder you want to share for uploading and click the "Direct-Upload-Link" button:
In addition to the standard checkboxes for a file upload, you can select a "valid until" date and an optional user name for analytics and logging purposes.
Once you create the link you will be redirected to the folder view again. In the top of the page you can see the direct-folder-link written out, with the note that the link was automatically copied to your clipboard:
The user who opens the direct link for uploading will only see the folder name and all files, which where uploaded using this specific direct-upload-link: