There are 2 main options when it comes to populating a 3D CGI visualisation scene wiht people. The simplest way is to add 2D images of people in post-producton. This can result in nice results when done correctly, however it can be sometimes hard to get them looking true to life if the visualisation is complex or the lighting is a little unusual.
The second option and out favourite at North MadeS Studio is to add in 3D high polygon models of people. This saves time on the post production and can result in much more lifelike looking people as the lighting effect and environmental interactions between them and the rest of the scene match much better. Realties shadowing is created and overall an ambient occlusion is created.