Hiya!
Don't make it a "rule", or the power-game'ey players will exploit it (or try desperately to). Make it an exception. Personally, I'd guestimate how much "space" the people/targets take up in the area targeted, then tell the player to roll to hit. If he rolled well enough, I'd roll % based on the rough percent of possible targets. So, if there were three guards standing around a fire in the middle of the courtyard... figure each guard takes up two square feet. Courtyard has, say, roughly 1100 square feet. So, six square feet vs 1100... waaaay less than 1%, so I'd just roll d100. If I got a 3% (my personal "minimum chance" for just about anything in my games), a guard was hit.
Seeing as there are no detailed rules for angle/arc, windspeed, humidity factors, etc in D&D, I wouldn't allow a "spotter" to guide the shooter any more than "Three guards, in the center, around a fire"; this info gives them the 3%. Without that, I'd probably drop it to 1%.
What I wouldn't do: make a rule of "Perception DC 20, then Disadvantage to hit". That kind of "solid, all-the-time" sounding rule is *instantly* going to get the rules exploiters grey matter churning for ways to somehow get Advantage on to his and perception, or reducing Perception by some ungodly amount, etc. Anything to get to the point where they can say "Oh, I can just shoot him at normal, after a DC 10 Acrobatics check because of [insert crazy list of things], which lets me use Acrobatics in place of Perception when I can't see something, and the DC is always 10 for that because of [insert crazy list of things]...and, as I have +8 to Acrobatics, I only fail on a 1. I don't have to roll Disadvantage to hit because [insert crazy list of things], so, yeah. ---rolls a d20, twice--- Made my Perception and I hit AC 19".
^_^
Paul L. Ming