The first thing to remember is that a plural never takes an apostrophe. A simple plural just adds an 's'. There are some irregular plurals, of course, but they still don't need an apostrophe.
A pronoun is a word that is used instead of a proper name. They are quite happy standing in for their noun so they don't need an apostrophe hanging around.
When you do need an apostrophe is to show that something belongs to someone.
If the owner is plural, it takes an apostrophe after the plural 's' because the object is belonging to them, not because there's more than one of them.
Some nouns are already plural, but the same rule applies.
Where it does get tricky is in words that already end in 's' and grammarians come up with different solutions. So follow whichever one of these guidelines you prefer.
Option one
One-syllable words take an apostrophe and the s.
Words with more than one syllable just take the apostrophe
Option two
If it sounds okay, do it
If it sounds like you are turning into Kaa, the snake, leave off the extra s