A few further notes here.
When starting out be sure to gather a good amount of reference. Do your research not just on what crowds look like when they're stationary but also how they move. Observe in public areas or watch a scene in a movie.
Movies are also good for reference because the photography will be stylistically stronger. You'll notice that the main character is wearing red (for example) while the other figures are lost in blues and greys.
Make sure your perspective is right. It's a great practice to get in to and using simple one-point perspective is enough for most crowd scenes.
When laying out your scene keep it lose and rough, especially in the beginning. Dwelling on the details of faces, when you haven't nailed your composition and values, will only serve to pull down your image. Use a large brush and if you like, allow colors and forms to merge into each other.
Remember the rules for creating depth - Distance = Desaturation, lighter values, cooler colors.