Well for one thing, the world is full of 'walk-on' characters, people like waitresses, car mechanics, fast food servers, grocery check-out folks, bank tellers, pedestrians, security guards, cops, nurses - well, you get it. A good place to get a feel for this is the movies you like. Play a DVD or get a download online and pay attention to the background characters. Most are background, literally, you see them move around, but that's about it. A few sort of cause something to happen or have a line or two of dialog, but are not a focal point in the story so they do what they do and then move on. So with this in mind, remember your writing is going to reflect that as well.
You'll need to write in minor characters, sometimes important ones (as backdrop goes).
What about these people?
Well, they need no more than one or two lines of introduction. Probably they don't need names and just a bit of description is enough.
Focus on details.
Maybe the kid on the bicycle is skinny with a "TinTin" haircut and oversized jeans. Maybe the burger flipper is tall, skinny, wearing bright yellow and has dirty fingernails. The PTA mother could have flaming red hair and a temper to match. Maybe the baseball coach is revealed to be bald when he jerks off his cap and throws it into the dust in disgust.
You'll need to create these secondary characters with your writing so they form a backdrop for your main characters and don't steal the show themselves. They are actually more like 'things' than actual 'people' though of course they must be portrayed as real, not cardboard cut-outs.
All this means you need to strike a balance. Don't get carried away and write in too much punch for your minor characters. Through these characters give your readers taste of the environment they, and your main characters, occupy. Your minor character might even have a line or two of dialog, but what you're writing is to push the plot along.
And at times all you'll show through writing and words is a crowd scene, or a scattering of people with no real defining traits. More backdrop. If you're talking about a stadium full of people, a street scene or the opening run at Macy's bargain basement, you're not going to write descriptive passages about all the people occupying those scenes. Just the main characters and perhaps a minor character or two. The rest create a crowd and backdrop.
You might skim a couple of your favorite books with all this in mind and see how the writers of those books handled their lesser players. And if you'd like to read a bit more about creating minor characters click here. But again, the article is about writing minor characters - not the ones who fill your written crowd scenes.