Apostrophes indicate either contractions or possessives. Possessives are indicated as follows:
- singular noun, e.g., "boy" - Add apostrophe s ('s) > the boy's desk
- plural noun ending in "s", e.g., boys - Add apostrophe after the "s" > the boys' classroom
- plural noun not ending in "s", e.g., men - Add apostrophe s ('s) > the men's cloakroom
In other words, a plural noun not ending in "s" is treated the same way as a singular noun. In an article about a bug in Facebook, the reporter for the Guardian writes:
Facebook could not say how many peoples' private pictures had been viewed as a result of the flaw (The Guardian December 8, 2011).
Now "people" can either be a plural noun meaning persons in general as in "the people" or a singular noun meaning a race or tribe as in "a people". In the latter case the plural would be formed by adding an "s", as in "Aboriginal peoples", and the possessive would be indicated by an apostrophe after the "s", as in "the Aboriginal peoples' charter".
But in the example from the Guardian, "people" is already a plural noun, meaning persons in general, and the sentence should read:
Facebook could not say how many people's private pictures had been viewed as a result of the flaw (The Guardian December 8, 2011).
Apostrophe errors are very common and one often comes upon huge properties or organizations belonging to or serving one person, as in Boy's Gym instead of Boys' Gym or Cross Lake Teacher's Society rather than Cross Lake Teachers' Society.