California Process servers are required to be registered in the county in which they work (California Business and Professions Code §22350 and §22353). Here is a link about the rules and regulations applicable to them: http://www.serve-now.com/resources/become-process-server/california
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Generally, I know of no "licensure" process for a someone serving legal papers - they merely need to be over the age of 18 and uninvolved in the case at hand. You can object to the service in court to the judge assigned to your case, but there really isn't an agency that handles someone serving papers.
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