You have four different ways you can add reviewers (in order of fastest to slowest)
Method 1: Let reviewers sign up on their own through the reviewer registration form (enable it from Website builder > Navigation) and entering their information. To make sure only invited users (Here's how you can send invites) are able to register as reviewers, you can generate a secret code (go to Contact > Contact settings to generate the secret code) which you would need to send to all invitees. To complete their reviewer registration, they would need to enter the secret code you sent in the invitation.
Method 2: Import reviewers from your spreadsheet (see this tutorial) and then send an invite to them with a welcome email template.
Method 3: If you have existing contacts in your system and you want to convert them to reviewers, then simply select those contacts and click on Bulk Edit tool to assign reviewer role to the selected contacts. See this tutorial.
Method 4: Go to Contacts, click on Add New to add a contact. On the left, click on Select Usertype dropdown to select the Reviewer usertype and to activate the account. Click on Save in the top right corner. That's it.