According to Fred Brooks, author of "The Mythical Man-Month", the practice of adding more people to a project at the last minute may not yield the results you want. From Wikipedia, this is known as Brooks's Law:
"adding manpower to a late software project makes it later"
Software isn't like manufacturing. When I was a lumber stacker, it took me a little under a week to form the mental connections to know which direction to twist to find the right stack for the boards that were coming down the conveyor system, and it took me about 3 weeks to where I wasn't in physical pain at the end of each day. :) The company had a lot of turnover, but we were also pretty interchangeable at that level.
With software, it can take good programmers a lot longer than a few weeks to really gain a solid understanding of the systems. Software is more complex, and requires an almost strategical, high-level understanding of how everything connects together. The longer someone has worked in the code, the more they know about where certain bugs, system flaws, and interfaces can be found, and this allows these developers to more quickly get work done and with higher quality.
In my experience, adding people to a project means those people aren't productive for at least a month. Sure, they might take a small bug or two that isn't mission critical, but until they gain a better understanding of how everything connects together, the only impact they really have is in reducing the productivity of the people who have been on the project longer.
If you know a project is going to be late, you have two options that are almost always better than adding people late:
Meet with the stakeholders and discuss cutting non-mission-critical features.
Push back the project deadline to account for the extra time needed.
By adding people late, you increase the chances not only of quality suffering but of the project being even later.