Can I volunteer or work without getting paid while waiting for my work permit/greencard?

There is no immigration bar against anyone volunteering or working without getting paid as long as there is no compensation whatsoever including payment for meals or transportation. I do note, however, that such a situation might be difficult for you and the employer to explain if there is a workforce enforcement action by ICE and you are located on the premises. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.