Outwardly, which means by the text from the book I’m using to inform these documents, this English usage worksheet on understanding the use of their and their’s addresses the problem, which you may have noticed, of using an apostrophe with the plural possessive pronoun their.
And the short reading for this worksheet does deal with this, but is also emphasizes the fact that none of the possessive pronouns require an apostrophe when used with an s, as in a construction like “The jacket is hers.” I’ve also built in some instruction on antecedent/pronoun agreement, which is one of those stylistic lapses that I can tell you from considerable experience–both as a student and a tutor–that lands students in their college’s writing centers.
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.