Employers must require workers to prove identity and eligibility to work within three business days of hire. An I-9 form verifies that a worker is eligible to accept employment in the US.
An employer and the worker both fill out the I-9. A social security number often proves identification.
Note that if the social security number does not match the designated worker, then an employer may later receive a “No-Match” letter. There is no immigration consequence. The letter does not prove whether the worker is undocumented or not. Also, the employer does not need to reverify status.
For the workers who are not eligible to get a Social Security Number, they may acquire an Individual Tax Identification number, or ITIN number.
In 2006 1.4 million people used ITIN when filing taxes, of which more than half were likely illegal immigrants. Opponents argue that ITIN numbers institutionalize illegal immigration. Supporters argue that our economy benefits when undocumented immigrants pay taxes.
If an individual is in removal proceedings, whether or not the immigrant paid taxes is a discretionary factor the Immigration Judge will consider, in totality with other factors. Matter of C-V-T-, 22 I&N Dec.7 (BIA 1998).
See a lawyer or accountant with knowledge of immigration issues if you seek tax advice related to immigration. A lawyer with an employment law background will help you hire new workers.