There are a couple of ways to create a realistic shadow, this is the one I like.
I have to assume you have a good working knowledge of PS.
Make a selection of the people using whatever method you're comfortable with, lasso, quick selection, magic wand, etc. it doesn't have to be perfect.
Use Refine Edge, smooth a little, feather a little, and shift edge in a little, set output to New Layer and hit enter.
If you have an old version of PS you may not have Refine Edge, in that case just use Ctrl+J (Cmd+J on a mac) to put the selection on a new layer.
Now duplicate the layer again.
What you should have now is 3 layers, the top two are just the people with no background, the bottom layer is the original with the background.
Turn off the top layer.
Select the middle layer
Hold down the "Alt" key (Opt on the Mac) and at the bottom of the layers pallet select "Create a new fill or adjustment layer" and select Levels.
Drag the left slider to the right and the image should turn black.
In that center layer paint black any areas that didn't go black
That's your shadow, rename the layer shadow.
Turn on all the layers and select the shadow layer.
Use opacity, Gaussian blur, and transform to get the look you want.
There will be areas of the shadow that you don't want, you can just erase those areas but I strongly suggest you mask those areas instead.
That way you can get it back if you change your mind.
With that shadow layer you can create soft subtle shadows, long harsh shadows, or anything in between.
I know you don't want anything like the second image, it's just an example using the same layer differently.
