People detection from above
Are there any other objects that can move beside people? If there aren't, you can just find the  blobs (connected components) in your foreground mask, and these are your people. 

They can also "collide" one with another, creating one blob instead of two. In this case, you can do a motion tracking and resolve the ambiguity by using the fact that the trajectory and the speed is smooth.

If there are other objects (like dogs, cars), you should create a classifier that gets blob parameters like:


Blob statistics (size, solidity,etc..)
Color
Edge information
Speed (in case of tracking)


And returns the correct class (Human/No Human).