User’s Guide vs Users’ Guide

The difference is in the number of users.

User's guide: A guide belonging to one user.

Users' guide: A guide belonging to all the users.

I'd be inclined to use the first sense rather than the second sense, since the "one" user is really an abstract representation of all the users anyway.

The difference is this: s' is used for plural possessive; 's is used for singular possessive; s' can also be used at the end of proper names that end in s to indicate the possessive without adding the extra s.

Since only one user is likely to be reading a "User's Guide" at a time, it would make sense to use the 's in that title. That may be why your co-worker thought it was funny; possibly he envisioned hordes of people trying to read the document at once.

It may just be my excessive mental flexibility this morning, but actually both make perfect sense to me.

In one case, we have the guide "belonging to" (i.e., "for the benefit of", and targeted at) the single abstract user who represents all the actual physical incarnate users. There is in a very real sense only one user reading this thing at a time, and it is that person that the writer is addressing.

In the other case, we have the actual physical users, who are certainly legion, and without which there would be no point in the existence of the Guide. It is to these people, collectively, that the Guide belongs.

I have written a lot of these things in my day, but to be honest I could not tell you now what I called them at the time.