The role of a reviewer in a project is often low-key. To reflect that in Content Workflow, we recommend that you reduce their Roles & Permissions so they really can’t do very much apart from review content.
Go to People & Groups in your Content Workflow account. Here you can create a custom role or edit the closest role to it, which is probably the editor role. The role can be edited to have permissions based on reviewing, rather than client permissions, which are based on editing content and producing it as well.
This removes the ability to write content and view all items in a project. If you remove the latter, then the person can only see items they are assigned to manually by themselves. This means from the reviewer's perspective they will only see items they need to know about and it means they can’t edit the content on these items, they can only comment and update statuses. These are the only two permissions they really need to have.