Apostrophes do not appear in plurals. Apostrophes do not appear in archaic possessives. Apostrophes are not toys, and should not be given to children under the age of six, or, evidently, many people who are in the business of making signs.
"There," "their," and "they're" all mean different things. "There" looks like "here," so you know it refers to place. "Their" looks like nothing on earth, so it's pretty clear that it doesn't mean "they are." "They're" looks like "they are," except without the "a." This is how we form contractions. It's not difficult.
"His" and "hers" have no apostrophes. "Its," when a possessive, has no apostrophe. These things are always true. You must expend the effort to learn this rule exactly once.
If a family's surname is Williams, the plural is not "Williams'." It is "Williamses." That may look goofy, but I can't help that. At no point is it correct in this case to write "William's." Have you ever found a word that admits the random insertion of apostrophes?
The only situation in which apostrophes are a little bit sketchy is in possessives of names ending in "s." The question then becomes whether to write "Thomas'" or "Thomas's." The answer is, essentially, "yes." I prefer the former, as do most pedants, but the latter, for consistency's sake, is permissible. So, really, there's no excuse. Just so long as you don't write "Thoma's" or "Thomases," you're good. And both of those are patently ludicrous.
Which means that this whole thing is just not that hard, and if you screw it up you are either lazy or inconsiderate, and it is up to you to decide which is worse.