@Riko Well, it's not like I've discovered some big magic secret and can do it effortlessly, but I found that one way to make it easier is to, as I said, layer the elements of the crowd.
I've got some crowd-scenes coming up in later pages of Grassblades, and in it, I've chosen to place my main characters in the middle of the crowd. That is, they've got people beside them, who are drawn with some detail - then they've got people behind them, draw with less detail, and partly obscured by the main characters. And then, finally, they've got people and objects in front of them, partly obscuring them.
Here's a quick sketch of what I mean: the two characters in the middle (the frowning figure and the small child) are my main characters. The white figures around them are either on the same plane as they are, or close to it. The darker figures at the bottom are closer to the camera, and the blue blobs in the background are further away, less detailed, to fill out the background and make it feel like an actual crowd.
The benefit of layering crowds like this is that since you're putting some characters in front of others, you won't have to draw as many characters to imply that it is a big crowd; you can block parts of figures with other figures, etc. You won't have to draw as many feet! XD
Other things to keep in mind when drawing crowds - people will most likely be moving in different directions. Some will have their backs to the viewers, some will be in profile, some will be facing the viewer. People will look at each other, or at buildings. And your main characters - if they're there - will be moving through the crowd. They won't just be standing around in front of it, or to the side. Putting them in the crowd means they'll be interacting with it, and it makes it feel as though the crowd is actually a crowd, and not just a background you've pasted your characters on top of.
