Grammatically Correct: The writer's essential guide to punctuation ...

the people's choice
the alumni's lounge
PUNCTUATION
Note that this rule means that words where the plural is the same
as the singular will have the same possessive form in both cases:
For example, the sheep's wool could refer to the wool of one sheep
or of multiple sheep. If you are ever faced with this sort of construction,
make sure that the context makes your meaning unambiguous.
FOR PERSONAL PRONOUNS, ADD JUST S
This one is yours.
That one is hers.
The house on the left is ours.
I'll do my share if they do theirs.
Put everything in its place.
A particularly common error is to confuse the possessive its (no
apostrophe) with the contraction it's, which means "it is" or "it has."
(Consider the difference in meaning between A wise dog knows
its master and A wise dog knows it's master!) Those who insist
that the possessive its take an apostrophe because a possessive
"always" takes one should take note of the fact that none of the
other personal pronouns take one either. For more on this, see the
discussion under "Pronouns" on page 248.
FOR JOINT POSSESSION, MAKE ONLY THE LAST NOUN
POSSESSIVE; FOR SEPARATE POSSESSION,
MAKE EACH NOUN POSSESSIVE
my mother and father's house
Arthur and Lisa's daughter
Robert and Martha's partnership
the Browns and the Murphys' vacation plans [if these families travel
together]
Montreal's and Toronto's subway systems
my brother's and sister's weddings
cats' and dogs' grooming habits
the Browns' and the Murphys' vacation plans [if these families do not
travel together]
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