If you have been sent a guest voucher from this site you have invited to upload files one or more times. The simplest way to do that is using the information contained in the invitation email. When uploading as guest, be sure to verify that any links in the guest invitation email are to a Terasender that is running on a research facility you trust. If you are not expecting a guest link from a facility you know then the email might not be legitimate.
The user who invited you to this system might have done so allowing you to upload files and get a link to allow other folks to download those files. If you can not get a link then you will have to provide email address(es) of the people who you wish to invite to download the uploaded files.