This question has two sets of answers: The Dresden Files RPG answer, and the general Fate based games answer.
Dresden Files has very few situational modifiers. Much of what you'd handle in other games with terrain modifiers, lighting modifiers, etc are covered by Aspects. That's because this version of Fate doesn't seek to model physics; it's attempting to model dramatic narratives, where things only matter when, you know, they matter.
Now, there are some rules for adjudicating non-Aspect modifiers. These begin on page 311 and wrap around to 312. The rules are a bit vague, they use the word "Aspect" more than I like, and they don't explicitly mention resisted rolls.
However! If you take a look at page 142, under the Hiding trapping of Stealth, there's an explicit mention of using the modifiers on page 311 as the basis for resisted rolls. So you can make your Stealth roll against a difficulty as modified by lighting conditions, then compare Shifts to, say, an Alertness roll.
So there are some modifiers. They can be used in contested rolls. They are pretty vague and are basically left up to the GM to use or not as they wish.
Now, the general answer for Fate games.
Some other Fate games have a more robust set of modifiers. Spirit of the Century and Kerberos Club (Fate edition, as compared to the ORE edition) both have things like lighting condition charts. They both offer more support for things like circumstantial modifiers.
However, whether and how often such things are used is mostly left up to the group to decide. If you think that there should be more modifiers, use more modifiers. I'd just suggest talking it over with your group so that everyone's on the same page.
As for the bunch of people question...
There don't seem to be rules to Compel a group of people with one Aspect. You can Invoke For Effect on scene Aspects, which I imagine you could do for something like, "There's no clear line of sight between us," or the like. I think that's the simplest way to do it with a group of NPCs all at once.
