Thanks! i'll definitely check that out!
I'm not so sure that he is using a crowd package, I don't recall any of them having Halloween party theme (I could be wrong of course).
Other options available are that you can create your own billboard characters and use them in a scene, I've taken that approach before in the past.
I also noticed in the images shown that there were no shadows being cast, so I suspect that in this instance it is just a case of layering images. Render the background, and then render the characters in multiple passes without a background, and then just layer the images in whatever post editing tool you decide to use. I've used this approach with animations where I wanted to have a lot of characters moving about within an environment; Although I layer those rendered files using Davinci Resolve, but the same approach can be performed in Photoshop or any app that supports layers. If the characters need to cast shadows, then that can be achieved by using an Iray Matte. Or alternatively just add shadows in Photoshop as part of the post process work.