The I-9 form must be completed for all employees hired on or after November 7, 1986, as well as for any existing employee who requires re-verification. You do not need to complete a Form I-9 for persons who are:

• Hired on or before November 7, 1986, who continued in their employment and have a reasonable expectation of employment at all times;
• Employed for casual domestic work in a private home on a sporadic, irregular, or intermittent basis;
• Independent contractors;
• Providing labor to you but are employed by a contractor providing services (i.e. employee leasing or temporary agencies); or
• Not physically working on U.S. soil.

See https://www.uscis.gov/i-9-central/complete-and-correct-form-i-9 for more information.