Employees listed in the Review Employees tab are not yet added to your Request Finance account.
These employees likely downloaded the Request Mobile App themselves and indicated in the App that you are their employer by adding the email address of your Request Finance account.
You can approve or reject employees in this tab.
Approving employees: the employee is added to your Request Finance account and can submit expenses to you.
Rejecting employees: the employee won't be added to your Request Finance account.