You can apply a mask to a composite with several elements connected to or to a group in the same way you would apply it to a single element.
A simple way to make a silhouette out of a character would be to have a black card covering it with an inverse cutter. You can also use the characters own matte, something that would work better if you want to uncover it in a more natural way.
I’m mentioning nodes and composites but that’s only applicable to Harmony Premium. If you’re using another version you have to do it through the Timeline.
There are many ways to make a silhouette with original image as mentioned above as one of the examples. You can also use the Shadow nodes by removing the transparency and set the colour as 0,0,0. And then add the Transparency below by setting it to be transparent over the time. And then the colored drawing behind will be shown.