I think kak33 gave a very good explanation of an effective way to bring up this topic with management. An employee needs to take the initiative to write out exactly why they think that additional headcount is needed. Like kak33 mentioned, if they have a detailed log of their day to day activities, highlighting tasks that they are unable to complete, and explain exactly what the resource is needed for, it will carry much more weight than just going to your manager and saying you need more resources because you feel overworked. From my experience, facts are essential for having these conversations, and not only that, but if you clearly describe the problem, show how it is impacting the project, and propose a logical solution, it will make your manager much more likely to act and get the additional resources that are needed.