Why? Someone agrees to no salary, what claim do they then have?
At the very least, there are hourly minimum wage laws that you cannot agree to forfeit.
Lest someone think this is incorrect, California anyway has "you must pay overtime exempt workers at least 2x minimum wage" (or they can sue). Start at the overtime page: http://www.dir.ca.gov/dlse/faq_overtime.htm and follow through on exemptions.
Source: have known founders who had to sign waivers agreeing not to sue.