There are a couple different reasons why a particular user may not be showing up in your list of potential reviewees.
1. Your user does not have the Employee 'privilege' assigned to their account. Only users with this assigned privilege can be reviewed. Go to the Users page and press Edit on the person in question. Be sure to check off the Employee privilege and press Save.
2. You've selected only a subset of your user list
In the example above, you can simply click the X button next to 3 selected and all users in the system will appear.