One of the best ways to add people and trees to renders in architectural visualization is with textures. If you manage to find textures with the proper alpha map or with a transparent background, you will be able to add these textures to a plane and use them on your project. This is a much better solution for architectural visualization compared to the use of polygon-based models. Of course, to use such textures you have to either produce them yourself or find a free colletion on the web. The guys from XOIO-Air just posted a collection of 2d cutout people on their web site that could be really helpful on some architectural visualization scenes.
What makes this collection so useful? All people on the collection are viewed from below. So, for those of you with projects where the camera is placed at a lower angle, may use those textures to add a sense of scale to the images.
free