Heya, thanks for asking! Organizing illustrations with more than three characters (heck, even just one) is really hard, I agree. Basic composition will still assist in this, so here’s a tutorial on that alone.
For a quick tip, triangles work pretty well for dynamic shots with a small handful of characters:
You can use other shapes too, it’s mostly about where your focus is, where you want the eye to go, is it a scene or is it just a group shot like a poster for a movie.
For a large multiple group, you can always look at paintings from art history! There’s a lot out there with large quantities of people in them, like this Caravaggio piece.
That composition shape, with the long line and two shorter offshoots, is really handy to build off of, too. There’s a great post about it on MuddyColors. You can use character heads as points, like in a constellation. Use gaze to show viewers where to look. Light focus can also highlight the area of interest.
As for negative space, it also helps to think about it, because you still need space for your eyes to rest a little. Sketch out thumbnail sketches first, small ones so you can focus on composition and flow. Shut your eyes for a few seconds and focus on the black inside your eyes, then open them and look at the composition with a fresh look. It also helps to turn it upside down; it’s said that a good composition reads from any view, though that’s not always the case. Also, it helps to ask someone how well it reads.
Starting with composition and thumbnails can save you a lot of troubles with negative space. You can see how the characters are arranged before you draw them, and you can identify large, weird gaps of space before you begin drawing for real.
If you struggle with drawing a lot of bodies in one spot, draw them individually, like on separate layers, and then later, mask or erase the parts that get covered up. If you’re working traditionally, try using a lightbox to trace over separate drawings onto the main sheet. It’s also a good idea to work background to foreground, so draw the background characters first and work your way up to the foreground characters.
I hope this answers your question! If not, feel free to ask for more info. :)