No matter what size your company, you must have an I-9 on each one of your employees. Recently, I have run across employers who do not have completed I-9’s on their employees.
What is the purpose of the I-9? The I-9 is required by the Department of Homeland Security, US Citizenship and Immigration Services for the purpose of documenting that each employee (citizen and non-citizen) hired after November 6, 1986 is authorized to work in the US. The employer maintains the form and it is not sent to any government agency.
You can get blank forms at http://www.uscis.gov/files/form/i-9.pdf. The forms should not be maintained in the personnel files, but separate. You will find the details of how to complete the forms at the website.