The Radiance sky/ground models that are automatically generated for you in OpenStudio will ASSume 20% reflectance. The big caveat there is that the "ground" is modeled as an infinitely distant hemisphere that simply reflects 20% of the solar/sky radiation. In other words, the building is magically suspended in this environmental, radiating sphere. In real life[TM], the building is sitting on the ground, and the building casts solar shadows on that ground, thus dynamically altering the ground plane reflectance. So if you want to properly model the ground reflectance in Radiance you need to add some exterior shading polygons representing the local ground plane. In this case, the reflectance is simply the reflectance of the material you apply to it.