I was in US for complete 2006 on L1. My family joined me in between for 3 months on L2 visa. I want to file tax jointly. But marital status in W2 is SINGLE. It may not be possible for the employer to change the W2 form since it is already too late.

Can I still file TAX jointly? Please suggest me.

Yes, you can file jointly. If you do, your spouses's world-wide income become subject to U.S. taxes.