Hello,I work for company A on L1B. Company B wants to file my Green card as future employee. when can I join Company B to work?

No.

I am an illegal immigrant married to a U.S citizen. Will I need to leave the U.S when applying for a legal residency

I agree with my colleagues. You might be eligible for the deferred action.

Can i travel to US with H1B Visa, which is expiring on 30'th sep 2012.

As long as the job described in the original H-1B petition is available for you, you should be able to enter the U.S. with the valid H-1B visa.

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Can you switch employer and keep the same PD if the original petitioner is no longer hiring?

I agree with my colleague.

I'm an immigrant employee. Should my letters of reference be almost identical to my job description for the PERM application?

Please address your concern to your attorney.

H1B visa

As long as you have a valid visa, you can travel. The extension application must be filed while you are still in the U.S.

TN Visa change of status after renewe TN-2 visa stamping

1. Submit the extension applications for you and your family. 2. You have to get your 1st TN visa from the U.S. Consulates/Embassy in Mexico....

"what can i use for document showing i was present on june 15,2012? can a letter from a friend or coworker help?

You can contact Houston Legal Aid Society. They might be able t help you.

Registered agent_international student

That is considered as an unauthorized employment.

What is my current immigration Status for form I-765 Application for Employment? I entered the U.S. as a child E.W.I?

I agree with my colleague. If you used someone's passport to enter the U.S., it is a different story.