We recently came across the use of a possessive apostrophe in a very popular lead capture SaaS site call Icegram.
So the question is:
Visitors, Visitor’s or Visitors’ ?
One could make a case on each of the three, if there was no context.
1. visitors: Visitors is a plural noun used as an adjective. Although seldom used in English, it is seen in phrases like “visitors gallery.” What kind of gallery? A visitors gallery.
2. visitor’s guide. If, as a singular visitor, I have a guide, then it’s a “visitor’s guide.” No plural needed because it is a guide for each individual visitor who uses it.
3. researchers’ document. This document was meant to be used by researchers. Not one but many researchers. No single researcher has exclusive use of it. researchers’ document.
Which is correct? Context is the only thing that would answer your question. Since there is no context, there is no “correct” answer.
But, in the context of the Icegram offering, there is context! Another example of English not being spoken proper.