Don't! You don't have to go out of your way to design elaborate or unique-looking people - just settle on a small number of basic looks, and then vary the details. Draw a chubby man in a yellow t-shirt - and then at the opposite end of the crowd, draw him again, but give him a blue t-shirt, a bigger nose and maybe a moustache. Swap colours and hairstyles, noses and skintones, etc. but give yourself only a handful of basic bodies to figure out.
Crowds don't need to be full of unique characters, unless your intent is to signal to the reader that this is a crowd they need to really investigate when they look at it.
If you want to make a crowd that says something about the place they're in, do some fashion research first. Is it a crowd of people in a very cold place? Look up how to draw winter-outfits. Is it a crowd in a city in a desert? Look up what people who live in desert areas wear. And so on and so forth. Incorporate a few details, but don't elaborate too much.