You can either create reviewers from your platform backend under Contacts or ask them to register from the event site by clicking on Reviewer Registration form.
Once registered, they will be able to log in to their dashboard.
As soon as you assign ans abstract for review to the selected reviewers, they would be notified of the assigned abstracts through an automated email notification.
They would click on the login URL in the email, log in to their dashboard and submit the review for the abstracts they have been assigned with.