Reviewers are recipients that will not sign the document. While they do have to approve or decline the document and can block the project flow, they will not sign the document nor be included in the final cryptographic seal.
Creating a project with a reviewer
First, following this guide, create a simple project and add a recipient. By default, your recipient will be a signatory. Click on the "Signatory" selector inside the step to switch the step mode into "Reviewer":
Now the step will be in review mode and all recipients added to it will be reviewers. As specified here, a recipient can be both reviewer and signatory, but not in the same step.
The rest of the process is fairly similar to any other project. The only difference is that a reviewer cannot have fields and, during what would be the "signature process", they will only have to approve or decline the document.