I am trying to do research on what can be done by unregistered people. While it is clear they cannot represent in a patent court. But to what extant can they help someone one write patents? or help others to file?
There is no such thing an an unregistered patent agent. Being registered is what makes you a patent agent. Registered patent agents can represent one before the USPTO and provide legal advice that is narrowly related to that representation. That means a patent agent can give a patentability opinion, for example. Patent agents can write patent applications, file them and prosecute them. That includes appearing before the appeals board. Patent agents can also handle the court-like process of IPRs.
Only a licensed attorney can represent you in an article III court (a real court). It is noteworthy that a licensed attorney who can represent you in a patent dispute can not represent you before the USPTO unless they have separately become a registered practitioner - like a patent agent.
Someone who is not an attorney and not a registered patent agent could do a patent search and provide patent strategy. They could provide education about the patent system. There are people who help others write patent applications and help with the mechanics of filing. I think they run the risk of practicing law without a license. Drafting claims for others is the practice of law since it requires the exercise of legal discretion as to what the proposed patent will cover. When the SCOTUS decided that patent agents could practice law without a state license (Sperry v. Florida 1963) in their very narrow area of law, they were agreeing with Florida that drafting patent applications and dealing with the USPTO to try to achieve an allowance was, in fact, the practice of law. And that drafting applications was one of the most difficult legal document to draft.
In a recent case the Colorado Supreme Court shut down a company that held it self out as "patent experts". The primary issue was that, while they did use registered practitioners to do the work, the company was run by someone who was neither an attorney or a registered agent and the company did not allow direct communication between the (qualified) people doing the work and the clients. This aspect of the case was a more general legal ethics/ULP issue. In most states a law firm must be headed by a lawyer. However, on top of that they found a case where the CEO himself finished a project by drafting claims for a client.
In response to a comment - Article III of the U.S. Constitution is the part of the document that creates the Judicial branch of the federal government. Patent infringement litigation occurs in that branch. The PTAB within the USPTO constitutes administrative law courts within the executive branch. They can rule on getting a patent as well as invalidating patents under the 2012 AIA law. Their decisions can generally be appealed to the regular article III system.
Anyone can write a patent application and the inventor can submit it to the USPTO themselves. If the inventor wants to have legal representation, they need to hire a patent law attorney or a patent agent; either one will be registered with the USPTO. Anyone can write a patent application for either the attorney or the agent, and then the attorney or agent will review it and file it under their registration number.
Many patent law firms employ a number of patent law attorneys, patent agents, and technical specialists. Attorneys typically have a scientific degree such as a MSc in engineering or biotechnology, as well as a law degree. Patent agents typically have a PhD, for example in physics or biology. Technical Specialists typically have a PhD, for example in physics or biology.
In many firms the actual patent can be written by either one of the 3 (attorney, agent or tech specialist), however usually if the tech specialist wrote the patent, the agent or attorney will review it before filing, because it's their registration number (and legal responsibility) that allows filing of the application. This review can amount to a very quick skimming of the text, if the tech specialist has been writing patent applications flawlessly for a few years. In fact, a tech specialist with a few years of experience will typically write much better patent applications than a freshly registered patent attorney, especially if the subject is very technical. The reason is that the tech specialists usually have a much deeper understanding of the technical side of a patent, since they typically have a PhD while the patent attorney will probably have an MSc. Once the tech specialist (or patent agent) writes a few patents (usually taking advice and correction from an experience patent attorney), they will have a strong command of the legal requirements of writing a patent, hence they will do a better job of an unexperienced patent attorney.
In fact, if the subject of the patent application is relatively complex, scientifically, a tech specialist or a patent agent (either one with a PhD) will very often write a better application than most patent attorneys. I should also add that many tech specialists (but not all) are really patent agents in training, in the sense that they are preparing to take the patent bar and become registered agents.
Often a patent application is based on a published peer-review paper describing an engineering invention. In these cases, the text written by the inventors forms the basis of the application. The attorney, agent or tech specialists goes over this text, modifying and adding as necessary, to prepare the final draft of the application.