Use RAMSession.put(IRAMSessionObject, IProgressMonitor) to register an unregistered user. Only a repository administrator can register users though the API.
//Register if not done so
if(!user.isRegistered()){
user.setName("Kevin Jones");
user.setEmail("kjones@example.com");
user.setPhone("555-5557");
session.put(user, new NullProgressMonitor());
}