This post is in response to a couple of comments, asking that I reveal all on How to Sketch Crowds. It's been a novel and slightly daunting experience trying to formulate what has come about through years of trial and error, and the good old 3 P's - practice, practice, practice - in my case both voluntary and imposed (when I was required to draw anything, fast, on demand as a junior 'renderer' in the ad industry way back when...)
Katherine Tyrrell has also done comprehensive posts on
drawing people and
people while eating - forgive me if I repeat some of what she's said - it probably means that's a pretty good point to remember!
First I'll presume that you're very familiar with drawing single figures and faces before you get to the crowds... lots of life-drawing, your family or friends in front of the TV, etc. Figures and faces from the side, front, top, three-quarter, back, sitting, slouching, smiling, frowning... Observation is a lifelong exercise that never ends - I'll stare at someone's foot, taking mental notes of how it looks from the front, or how a mouth curves when facing up and laughing... it's why I sometimes don't hear what you're saying to me, but also why I think I've got better at drawing over the years!

OK. Crowds. I start with one figure - generally around the centre foreground. I might start with sketchy pencil lines like this --> to vaguely place the figures, especially if there's perspective involved, but not always. I've numbered them more or less as I'd start and finish. If there are structure/s around which people are arranged, like chairs, tables, pillars, indicate them to give you a point of reference for size and position. Not too much detail on those - you can add that later if you want to.
With these at the
licensing department I sketched in pencil and added pen and watercolour afterwards at home, but keeping, as much as I could, the spontaneous feel - I wished soon after I started, that I had gone straight in with pen, but it was one of my first attempts at a crowded room, so I was a bit wary. And I didn't know I would have
so much time to draw!

I don't worry about getting the right people in the right places - who would know? - in this situation, every ten to fifteen minutes the queue of people got up and shifted 5 or 6 seats along, so the next person I drew would be relative to the first, but wasn't necessarily the person that was sitting next to them at the time.

I drew the head and shoulders of this guy (1), and the line got up and moved, so I left his lower half until a suitable body came along to complete the figure. The owner of the hands filling in his forms (2) actually had his head (with long, floppy hair if I remember correctly) bent right over them, but added onto my original head, they're just paused in their writing task.
A note on hands - don't stress over them, if they're the right size and vague shape, they'll register as 'hands' - don't get into counting fingers as I used to do!
I'll continue with figures to the left and right of my first one, front to back - I like to do two or three character sketches, just because I enjoy people's faces, especially if there are interesting expressions, hats (3), hairdo's (4) and so on. Then I fill in the gaps between the main players...whatever can provide some interest to a sea of heads and shoulders, I look out for and put in. A hand gesture to the face (5) a yawn, a stretch - anything to add mood or liveliness to a scene.

In this
Food Court sketch I think I started with the girl with the ponytail, as she was a 'safe' distance away not to notice. The woman on the right arrived and smiled and laughed continuously the whole time she was there, but by the time I got to her daughter, I think it was that she was talking to, they were moving off, so she's smiling and chatting to a total
stranger who sat down there next, and gulped his food so fast I only just got him down in time!

In this sketch of a moving, singing, dancing
choir, I started with the central figure of the choir-leader - I went straight in with my fine pen (0.2 unipin) - don't worry if lines overlap or if you do some 'wrong' lines - it all adds to the impression of movement and action, and might help in developing your own special signature style. I picked out just one figure that I had a good view of and sketched them doing whatever it was at the time - the one to the right of the conductor dancing, the next clapping, then to the gaps in between, going back row by row and filling in - again, nobody will know or care if you left somebody out or put someone in a different place, as long as they are more or less in proportion to each other.
As for colour - I mostly add it later at home, unless I have unlimited time and space, don't mind who's watching and have all my equipment to hand, which isn't often - in fact it's usually from the shelter of my car that I occasionally paint people on the spot. Sometimes I'll pencil in colour notes to remind me, as in the second pencil sketch - sometimes one or two striking colours serve to highlight a figure or group. With the choir, they were all in black but I used blue to enliven the darks, and their spotted scarves to further convey their energy and sparkle.
I hope you find this helpful, Linda, and any other interested parties - it was Cathy of
Cards and Stuff who commented "I'm amazed at how many people you can sketch !! And the sketches seem to have been done calmly... Are people sitting nicely still for you to sketch them or what? ;-) What's your magic trick?" No, I'm definitely not always calm Cathy, and they don't sit still, but hopefully this explains some of my process, which undergoes reviews constantly, so check back!