If I was doing it I would set up a separate render file just for the mask, in that file you set the background to 100% flat white, put a 100% flat black on the people and the floor to 100% reflection, when you render that you should end up with a totally BnW reflection to use for a mask.
I usually do the white in a luminosity channel, drop out all the lights and it should render in seconds being only BnW.
Deck