Crowd scenes can be tricky!
There are a few techniques to make them look good and also make them easier or more fun to draw though.

My latest technique is to do cameos of other people's characters- that saves on having to make up generic figures and faces and makes drawing a big group of people WAY more fun and interesting the the slog you have with a bunch of random non-character bodies.

Silhouettes in the background is good way to bulk up the crowd and add depth.

Something I've heard of but never really tried myself is to do groupings within the crowd or recognisable figures in amongst figures who're just the outlines of shapes- this can save on work and give your crowd a nice big size while also looking natural.

You can always try and draw every single person, coming up with a whole heap of totally unique characters JUST for the crowd scene- I used to do that myself. It's an AWFUL way to go.
Like this:
http://www.theduckwebcomics.com/Pinky_TA/4766806/
and this:
http://www.theduckwebcomics.com/Pinky_TA/4766809/

Actually I STILL do that:
http://www.theduckwebcomics.com/Pinky_TA/5474914/

I hate to say… I largely deal with it by avoiding drawing crowds whenever possible… I'm really looking forward to tips from the pros here!

Photoshop is your friend. Copy and paste. Make a small variation. Copy and paste. Repeat as necessary.

I do cameos sometimes (asking for volunteers), but not too often for a few reasons. Cameos don't always hit the demographics I want. I also fret over doing characters justice or them not being recognizable due to distance or whatever. Also, I either ask for cameos and get no response or ask before I get to the storyline and it's months later, and the person has gone on hiatus or vanished and can't enjoy it.

I like to use original designs for “extras” even if it takes effort. I avoid crowd scenes and use visual tricks (angles, close-ups) to create the “crowded feeling” without having to draw too many people.

Showing people from behind and only in portions makes easier art and you have the feel of being a section of a big group (cameos here too): http://www.theduckwebcomics.com/Used_Books/5454363/

Portions of lots of conversations is one of my crowd “tricks” :P :
http://www.theduckwebcomics.com/Used_Books/5412265/

Also here. (Very few characters, but it's still crowded.)
http://www.theduckwebcomics.com/Used_Books/5354598/

I'm not sure when I started giving crowds dialogue, but it makes the atmosphere I'm going for.

Lady Unlucky has a LOT of crowd scenes, and a good number of them I draw entirely out. I do a lot of cameos too, but designing extras has never been much of a problem for me. If all else fails I just throw in characters of my own that I don't use anymore.

One of the things I do to help with these scenes is sketch random stick figures for where the individuals are going to be, then try to give them snapshot backstories based on what I see in these stick figures' gestures. It's a lot easier if the foreground characters are doing something that will attract attention but if that's not the case then I can find interesting things for the extras to do. I think it gives more life to the crowd.

I do take shortcuts every once in a while, though, and for crowd scenes I do the same as what I do for background shortcuts. I'll paint a vague impression with a hard brush and blur it.

Well…. 3d leads to different issues to hand drawn.

The figures are ram intensive so there's a limit to how many you can have in a scene. Also you need to worry about posing etc. There are cloning tools in some software but I haven't used anyour as I usually need a varied crowd.

One option is to make the scene up separately and composite characters on it, but this can be an issue trying to get the angles right, match lighting and render large amounts of characters. I've done this a few times, but it can be a pain and more hassle than its worth.

What I've done on some pages for the coming chapter of Lore is make a plain with a 2d image of people on it and usedo transparency maps to make the rest of the plain invisible. This isn't perfect, but for low light background it works well enough.

In this rendering of a concert space, I treat crowds like a texture. I tried to maximize is the idea that a texture obeys the rules of perspective. Figures close to the viewer will have a lot of detail, and figures farther away will just become a sea of blobs. The reasons for this are twofold: it maximizes the illusion of space, and it minimizes the amount of work that I actually have to do, only drawing a few fully designed characters. However, the crowd in the distance shouldn't be a homogeneous wash. There should be islands of detail and non-detail in there to give the crowd a sense of organic chaos. (the image is easier to see if you open it in a new tab)

FC nails it again!! (takes notes)!

Shoutout to CDMalcolm's page here. I was going to say it's the most extras I've ever seen in a single comic page, but Fallopian Crusader's panel above actually trumps it. O_O

Personally it takes me forever to draw a single person so if I ever have to draw a crowd scene, expect massive amounts of cheating. xD

Blatant copy-pasting is blatant. But copy pasting then tweaking specific details like hair styles and limb position goes a long way. As does hair, skin and clothing colour.

Of course, I still have trouble getting individual characters to look right and good. Artists who have that nailed can probably draw a crowd faster than it would take me to. edit one into existence.

EDIT: Okay, Ironscarf is my new hero…

I kind of echo what Whirlywind said. I'll start by thinking about the situation - what kind of crowd it is (riot, rock concert, families at an event etc) and also crucially important, where do I want to place the viewer in the action. Once I've established that, I'll start to sketch it in using my good friend, amorphous crowd girl/guy. Actual features come later.

A crowd shouldn't be boring. It's an incredibly dynamic, exiting and constantly evolving thing, made up of individuals and groups of various sizes interacting with each other, all with their own goals and agendas. People close by might have noticeable interesting features, like the tattoo on that woman's shoulder, or that Donald Trump hair creation in the corner of the panel. Set back slightly, a group of people might be having some kind of disagreement, such as argueing over a map if we're at some kind of tourist attraction. To their right a gap has opened up and a couple of people are pushing through, keen to get somewhere.

Slightly further back a whole group of unrelated people are all looking off panel in the same direction - we wonder why. But the kissing couple haven't noticed whatever it is, because they're caught up in the throes of new romance. The guy staring at them doesn't notice the pickpocket who's lifting his wallet. Behind these people it's getting harder to see what's going on and details are sketchy, but as FC noted, there are still defined areas and spaces - the crowd dynamics are still happening.

By this stage (quicker than it sounds!) it's a lot easier to visualise who these people are and what they look like. I'll probably search out some references for the type of crowd I'm drawing, because there are only so many jackets in my head at any one time and I don't want them to look like thay all shop at the same store.
Or if it's my comic, my initial lines will suggest Groucho Marx, a Leopard Lady and a blazing wicker man and I'll just draw them in regardless.

Continuing with Photoshop as your friend, there is also multiple layers composites. Rather than do everyone together do small groups and put them together. Now if you save each group as separate images you can build a library that you reuse by rearranging the elements. I learned this the hard way. Never try to do a comic depicting mass battle scenes or company sized military formations. See my comic Go a Viking for the former and Attack of the Robofemoids for the latter. Never again all at once. Bits and pieces to put together.

Just as a nerdy footnote for those interested in the realm of 3D technology: There are procedural crowd-generating algorithms available for many 3D packages these days. The Unreal and Unity game engines have these capabilities. Just enter parameters, and random/semi-random people and behaviors will be generated. The upside is that the two software packages I listed are free. Unreal doesn't charge royalties for not-for-profit projects like webcomics, so there's no copyright problem there. The downside is that the learning curve can be quite steep.

That reminds me of one of the worst crowd scenes ever: All the demons in hell jumping up and down in a scene from the Spawn movie:



That movie had some good CGI, bu mainly a hell of a lot of BAD CGI. I thought this was a prime example of the latter, but looking at it now it's obvious it was just a bad copy and paste job using standard 2D footage.

Yeeeech. A bad compositing effect is as old as film special effects CGI or no. You want to see great crowds watch the movies of David Lean and Cecil B. DeMille. What appear to be huge crowds were often carefully done composite shots and or padded out with dummies. Look at the panoramic battle scenes in Bondarchuk's Waterloo. Dummies or brightly dressed guys on a hillside for flashes of color and nothing more. Precisely what you would see at the distance portrayed. What is realistically seen at the distances you ate portraying can make life so much easier as FC showed in his post about a crowd as a texture.

I still haven't learned how to do crowds well; I just create tons of 3D “extras” and put them in the background and render. It's painfully time-consuming, so I try to avoid crowd scenes whenever possible.

I think It's usually wisest not to put too much detail into a crowd, because you don't want them to distract from the main focus… but at the same time probably my favorite crowd scenes are the ones by Geof Darrow, which are always insanely detailed. I first saw his artwork in Frank Miller's “Hard Boiled.” There's so much insanity going on in the backgrounds of every panel that it almost overshadows what's going in the rest of the comic!

Heh, I actually came up on this issue just recently. I solved it by drawing the crowd from a distance, and used really simple stick-like figures that had less and less detail the farther away they were from the foreground.

I actually like drawing crowds. My core cast is very similar looking for reasons, so I like the chance to draw other body and face shapes. I get to step out of my comfort zone which is always good.

I agree, comfort zones can be boring after a while. If you can find ways to enjoy drawing crowds, the crowds you draw will be more fun to look at too. There's no excuse for boring the reader (except with really slow update schedules).