Creating a section for uploading files for visitors by invitation

On this site I want to have a place where visitors can upload files when invited. They might be several GB in size. I will provide the visitor with a password. The site will not be open to the public, it will only be used for this type of file transfer.
Conditions are that:
1. It needs to be with a login function, I will provide the visitor with a password, once they are invited to upload;
2. Preferably the visitor is not able to see what is already there (uploaded by others);
3. I do not mind if they get sorted in the regular library where I normally store pictures appearing on the website;
4. I need to have aces to the files once uploaded by logging in as admin.

My question is if anybody knows if there is a plugin for this or how to implement this in an easy way. I am not a coder, and otherwise I would say I am medium knowledgeable in WP.

Thanks.