To manually assign an application (or multiple applications) to one or more Reviewers, check boxes next to each application and click the Group Actions button that appears at the bottom of the screen. Then, you will click Assign.
A new window will appear where you can select the Reviewer(s) and click the Save Assignments button at the bottom. Now these Reviewers can login and see the applications they've been assigned to. If reviewers have their email notifications turned on, they will receive one email digest of the submissions available for their review.
On the Admin side, you'll find it helpful to add the Manual Assignments column to your table. This will allow you to easily keep track of who is assigned to each application.