If the compound noun has a hyphen or hyphens, the -s usually goes at the end of the noun, such as in secretaries-general, passers-by and fathers-in-law. To make the possessive form, simply add an apostrophe -s to the end...
Hey All My version of Excel 2010 has started automatically putting a single apostrophe at the start of every cell I type in except numbers and I can't work out how to stop it :-/ I've only noticed because it's started effecting code in one of my books where a cell gets a value ...