Crowds for videogames

Hello guys and girls, I am currently working on a videogame and we need to put some crowd in our arena, I am wondering wich is the best way to do this, we want to take profit about our characters but maybe they have more bones than we need for crowds. Please if you know a way or some workflow share with me. I have to be carefull with the performance.
Thank you!

From most racing games, the crowds are usually made 2d.

In the Particles Live Stream a while back they simulated a crowd/horde with a particle system: