Oscar (and Andrew and others):
(The lid is off the worm can ...) Scroll to the bottom for the short answer, or read on for the reasoning behind it.
"Possession" or "possessive case" doesn't necessarily imply simple "ownership" in the common sense of that term; rather, it can imply a whole range of associations
: e.g. "the boys' school" is a school for
boys, but is not owned by them; "the car's shine" is not possessed by the car, but is a quality
of it nonetheless (an inanimate object can't own things); "Reader's Digest" is a digest for
the reader, not possessed by her/him; "the dog's owner" does not imply that the dog possesses its owner, but there is an implied association
(on the other hand, a cat always
possesses its owner!).
So it is,
I'm afraid, necessary to have the apostrophe in our "owners' group" - unless we wish to risk the ridicule of pedants (me - a pedant!
). And I assure you, it would happen. One observation I've made over the years is that generally (there are of course exceptions) the level and style of written English on M-B forums is more "aware", more "educated", than the average on forums for commoner marques. I make no value-judgements by that statement, but merely pass on my observations.
Sometimes you'll see no apostrophe in the official name of an organisation: e.g. "Lismore Workers Club" (where in ordinary usage - where one's not referring to an official name - it would certainly
be "workers' club" - but that's because organisations, businesses, churches, etc. may spell and punctuate their names as they choose (regardless of traditional or "correct" usage). Most such apostropheless names are, however, the result of ignorance in the first place; they just can't be affordably changed once registered and operational.
Another usage that often leads to confusion is that of "sports coat", "sports car", "sports venue"; these, however, are not "possessives", but rather "sports" in such instances is acting as an adjective: a coat (or car, or venue) of the type
"sports". No apostrophe is necessary.
That reasoning cannot
be applied, however, to "owners" in the sense we wish to use it, because the "use it adjectivally" (as in the "sports" example above) does not
apply to people or people-describing words - into which category falls our "owners".
There's rarely a simple, straightforward answer in language usage - and logic, in the sense we understand it - rarely applies; this is, after all, English, the most illogical language in the entire world's collection of 6,000+. It is also the most expressive by far, with a basic
lexicon of over 600,000 words (extrapolates to closer to two million when variants are considered); the next richest lexicon is French, with about 150,000. It's because we have more words for expressing finer shades of meaning that we need fewer of them to say the same thing a French or an Italian person would.
Anyway, back to the question at hand: Yes, it does need to be OWNERS' GROUP.