Go to Administration --> Review --> Review Assignments:
Choose a paper in the list and click Add/Remove Reviewers:
Then, you can search the reviewer by name or organization (Note that the number in the parenthesis means the number of review assignments that reviewer has in the current conference) and click the name to assign the reviewer to the paper.
If you cannot find a reviewer in the list, you need to invite them as PC members first: https://conferency.zendesk.com/hc/en-us/articles/115003854171
Once you finish the review assignments for all papers, you should click the following button to notify the PCs about their review assignments: