There are generator scripts available in the market place, just search for crowd.
If you have Poser pro 11 the "combine figure" and "reduce poly" commands can be used to quickly produce a low poly figure.
An appropriately placed mirror can be used to create the illusion of more figures
This is an example why instancing would be useful in Poser, programs like Vue and Cararra do this kind of work very efficiently. Its possible to write a script to do it in Poser but the problem is once the figures are generated the program becomes unusable with everything visible. I tried to get around this by using low poly place holders which a script then replaces with a figure but it was still very clunky.