Most of the time, the more people you have at a dinner, the longer it takes. Except happy hours. When just two people are out for happy hour, there is no possible way to have just one drink. As that number increases to three or more, the happy hour will be much shorter.
When two people are out drinking, they will always drink at different rates. As such, the bartender will ask if one person wants another drink before the other has finished. Then the other person finishes, and doesn't want to leave someone drinking alone, and decides to get just one more.
And it continues until an external force breaks the cycle (train, significant other, dog, etc.).
When there are more than two people, most of the time there will be enough momentum to disband if one person decides to leave.
In this way, the optimal happy hour size for getting home before 9pm seems to be three. Of course, who wants that?