What do you call someone who is affected by an action, whether positively or negatively?

If someone benefits from an action, you might call them a beneficiary. If someone is harmed by an action, you might call them a maleficiary (although that's not a common word). For example, "The charity's beneficiaries are local homeless people"

Is there a word for the whole category of people who might be affected (either positively or negatively) by an action?

The context in which I'm trying to use this is "It is important to think of your business's most important groups of ____, and whether they're being helped or harmed".

"Stakeholder" is similar to this meaning, although it has connotations more of people who are interested or involved in something formally, rather than people who are simply affected by it.

Solution 1:

You call them the affected— as a noun, describing the person. That is the word of choice employed by persons who prepare legal documents filed in a court-of-law. It the proper word choice for characterizing someone who has been influenced positively or negatively.

It is generally used in a question posed to determine the impact or degree an influence has had on the subject, the questioner being presumed as already knowing nature of the influence.

For example: "How are you affected by the accident?" This question implies that an accident influence the subject negatively; however, it leaves the degree of injury caused to the subject ambiguous.

That's where the fine line is. So, to leave the nature of the influence ambiguous, you simply describe a person affected, and, like all words, used it in a context that does not hint one way or the other.