What do you mean by "users" exactly? It would help if you could describe what you are tring to do.
I am asking, because in R3 there is a difference between logins and records related to users. A login only serves for authentication and is managed inside the admin panel and/or is imported via LDAP. user data is managed inside individual applications and is filled with information relevant to the application.
user data can be created at will and also be connected to logins via APIs by executing backend functions. There is however no supported way to manage logins via API or backend functions.