We generally advise against allowing anyone who isn’t on ‘your side’ into a project.
You can add any users you want to a discovery project, either with ‘full’ access or ‘guest’ access. Guest access prevents a user from editing a project, and you have the ability to restrict which repositories and/or folders a guest can view.
However, a project may contain privileged documents, unredacted versions of documents, Re-write! notes and issues etc. that you typically wouldn’t want the ‘other side’ to see. Even with restrictions to specific folders, guest users will still see things like the names of tags and issues folders and also the names of privileged documents if they are in a folder they have access to. They won’t be able to view the documents themselves, but they may see the names of folders or inside the folders will see the list of folder names. These may reveal information you don’t wish revealed.
We recommend downloading any sets of documents you wish to share with ‘other side’ and provide them via a sharing platform such as Dropbox.