The chances are that the background will also "hold" similar colours as the people: Reds, Greens Blues . . Now if you had a monotone BG which wasn't within the people's colour range then you could have a stab at it. there would even be a possibility to do this if the Levels of the BG were sufficiently different. Another way would be to use the secondary colour corrector to select colours from the FG to be picked - the RED rose on a brides monotone dress and BG - but here, with much similarity in foreground and background?
Another way, which I have been experimenting with lately is to use the Difference option. Luckily I had a locked-off shot and had some static footage of the scene WITHOUT people walking through it. I then used Vegas to apply a Difference compositing mode to compare and separate the BG. I could then apply a further FX to another BG and had real people walking down a kinda cartoon staircase. But I'm not thinking you have this option available to you?
I've run out of options/ideas. Again, try the secondary colour corrector and grab a whole bunch of the bg and see if that masks nicely . . .
Then there is Bezier masking OUT the BG and floating this OVER a monotoned BG? Depends how accurate you wish to be?
Grazie