The possessive "s" ending is in fact the Old English genitive case, which was not written with an apostrophe - nor is it in modern Germanic languages. However, some Tudor or Stuart grammarians, who weren't half as clever as they thought they were, decided that, for example, "The Kings Arms" was short for "The King his Arms", and would even write it so in full when trying to show off. (The fact that they would also write "The Queen's Arms" shows how carefully the theory was thought out...)
Nevertheless, the existing rules on apostrophe use is fairly simple (aside from ludicrous rules where it is left out to complicate things, such as "yours" and so forth).
Leaver's Assembly - Referring to each single individual separately, it is each individual's assembly.
Leavers' Assembly - Referring to all leavers as a single group, the assembly belongs to them all.
Use whichever you matches the meaning you wish to portray, I'd use the second option.
Yup (he wrote, grammatically) number 2 is correct for several leavers, whose assembly it is.
Strictly, it should be St James's Street, as with all singular nouns ending in 'S', though I doubt whether Westminster Council has it as such on street signs, and few people bother to write Jones's rather than Jones'. and St Andrews, home to the Royal and Ancient Golf Club, is always written without an apostrophe. The Club and local people insist upon that, but what does the Club know of rules?
Although not written in stone, general practice seems to be that when a name ends in 'es' then 's is added if the vowel is unvoiced but ' if it is voiced. Thus James's book (unvoiced es) but Achilles' tendon (voiced). To write Achilles's tendon would lead to it being pronounced Achillesez, which is ugly.
Sir William Palliser, who developed the estate, named Barons Court after his estate in Ireland, which was called Baronscourt. As an answer, that is unsatisfactory though apparently true. Why wasn't his Irish estate called Baron's or Barons' Court rather than Baronscourt?And why didn't he put an apostrophe in the new estate's name ?
Love apostrophes, I was a great admirer of Keith Waterhouse's Association for the Abolition of the Aberrant Apostrophe. Where were camera phones when I saw a few years ago a brand new Merc Sprinter in Stockport, lavishly signed, indicating that it was carrying Wedding Dress's.
My butcher used to have me proof read his new notices, ever since I pointed out "lamb chop's" and "rump stake".
I really want to carry a big red marker round town, and use it!
A superb example of an aberrant apostrophe quoted by Keith Waterhouse was a hand-written market sign he spotted at the time of the marriage of Prince Andrew: "WEDDING SOUVENIR, ROYAL PEN'S" - with the apostrophe in question being an unfortunately long one...