I have two questions.

I have been working in US from July 2006 with L1 visa. My family joined me in the end of march 2007 and they hold L4 visa. In the W4 form I mentioned me as Non resident Alien and my status as "single".

Can I change my status to Married and resubmit W4, then what is my resident status(resident alien?) in W4.
Can I apply W7 form before submitting tax returns for my dependents.

Yes, you can change your status to Married on the W-4 and resubmit it.

No, you cannot submit the W-7 by itself. However, you CAN submit the Form W-7 with an amended tax return (Form 1040X) to change your 2006 tax return to Married Filing Jointly.

Contact me by email for details.

If I can resubmit the W-4, what should be my residential status, Non Residential alien or Residential alien. I am from India and having L1 visa.

Can I amend tax return for 2006, as my family was not with me in US during 2006, they joined me in March 2007.

Your spouse's physical location is irrelevant for filing jointly in 2006. She/he could be on the moon for all it matters. Just aas long as your spouse agrees to be treated as a resident alien for all of 2006.

Further, your spouse's residency status is imputed to your children, so you can claim them as dependents as well.