http://www.baument.com/  has a set of low poly people

The vendor 'azl' has a lot of 'crowd' paks here in the marketplace - grat sets!

You could also just render your scene in layers. 

Render the background with a few people and save as a TIFF or JPEG. Then add that render as your background picture. Then render again (over background picture) with the next closer set of people and repeat the process as needed. I don't know about Poser 7, but in Poser 6 it wants to always refit your background image so you have to make the U and V one (1) and zero the offset numbers so it matches your render output.