I created a crowd scene in this animation (from 05:03 to 05:30) http://www.youtube.com/watch?v=ZKIDR7njbXQ
For that, I animated a group of people into poses randomly selected from the Poser Library, with their keyframes randomly set. Then, using the process Greymalkn mentioned, in my video editing software, I added (overlayed) multiple copies. But, to prevent them appearing the same, I used the software to create a mixture of forward and backward motion, and a standard and mirrored image.

However, if Figures are expected to move around each other in a 3d world, simple overlaying is not possible (or not practical). The Computer running Poser must do all the work (and thus affect the speed of using the app).

And things get harder if the Figures are to move around. The image below is from my next amateur animation. It shows some of the 100 Figures who, after walking down their own Walk Paths are lifted upwards (as in 'the Matrix') using PoserPhysics2012.