Megan C. sent along this photograph of a sign with a correct apostrophe. (Correct! How strange for us!)
This sign is for an association of musicians, plural. It is, therefore, a musicians' association. We press our hands together repeatedly to make applause! Bravo! Bravo!
(The reason there is no "s" after the apostrophe is that you don't pronounce that letter. It isn't musicians-ez association. When you do pronounce the letter, "the bus's wheels," then go ahead and use the "s." Or don't--plenty of style guides argue for the omission of the "s" when making plurals of words that end in "s." Just be consistent.)