From the Hotline: Employment Without a Social Security Card
Question: Can an intern begin employment before her social security card has arrived?
Answer: Our very strong advice to you is NOT to allow the person to start work until she can provide legal documentation certifying the legal right to work in the United States (even for temporary internships). If the individual has been granted permission to work and is just waiting for the actual Social Security Card, she could contact the Social Security Administration and request formal documentation that could be used until the card arrives. However, please note that only a few nonimmigrant classifications allow individuals to obtain permission work in this country without an employer having first filed a petition on their behalf. Such classifications include the nonimmigrant E-1, E-2, E-3 and TN classifications, as well as, in certain instances, the F-1 and M-1 student and J-1 exchange visitor classifications. You do not want to get into a situation for a temporary internship (when there are probably plenty of available candidates with the legal documentation to work here) where you hire an undocumented worker. You should ensure that the intern has all of the necessary paperwork for completing an I-9 form, including work visas and proof of right to work in the US.
For more information, visit: http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/?vgnextoid=13ad2f8b69583210VgnVCM100000082ca60aRCRD&vgnextchannel=13ad2f8b69583210VgnVCM100000082ca60aRCRD