How do you find out who the investors are in a U.S. stock? e.g. how ownership may be concentrated?
Companies absolutely know who ALL their shareholders are. Ownership is filed on Form 3/4 and in 10-Q/Ks. Look there.  Guidelines for required disclosure are as follows: 1) Individuals must disclose when their ownership exceeds 5%; 2) Non-individual legal entities (read: companies; e.g. a hedge fund) must disclose when their ownership exceeds 10% (Form 13-F); and 3) All Officers and Directors Notice the word "required." For example, a entity (individual/company) may file "confidentiality letter" (which allows them to delay disclosing ownership) with the SEC as they are building a position.  So at any given point in time the information that is publicaly available may not be "up-to-date." And in all cases beneficial owner(ship).