Select the ground. Go to material setting advanced tab.  Make sure "shadow catch only" is checked.

If you don't want to render the ground shadows, select the ground. Go to properties, and uncheck "visible".

I don't think you need to change the background color. But I often do change it to match the color of the destination background (if there is one) just in case there's a bit of a fringe.

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.