Someone--I think it may have been Steve McConnell--countered Brooks' law ("Adding people to a late project only makes it later") by observing that by the time most projects know they're late (or by the time project managers recognize and admit it), the situation is actually worse than people realize, so you might as well add people right away. Ramp-up time will be absorbed into the chaos.
That idea rings true for me, at least for big, lumbering projects that I've seen get out of schedule control.
Small, Agile projects are a different matter. When a project can say "at our current velocity, we estimate that we can complete X points of work, but we're signed up for X+delta" that's a situation that's in control in a planning sense. There's probably data available to predict what adding bodies would do. (My rule of thumb is that you don't add people to help you this month or next; you add people to help you in three to six months.)