... making sure, of course, that you have nothing in the scene background that will render, such as a skydome or backdrop. The Poser 'Background' itself does not render.

If you want to render ground shadows but not the ground itself, then go into the Material Room, select the GROUND prop, and tick "Shadow catch only".

If you get some fine or transparent edge details, especially hair, showing up grey against your background, then change the render settings to render against a black background, and that should improve it.

Happy rendering, BigDave!

(edit) x-post, sorry!

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)