I beg to differ. In this case the grammar belongs to the aforesaid people and should therefore be people's.
I'm also sorry. The "Grammar" belongs to all of the people, not just the aforesaid people. Stop trying to oppress us, you fascist thug.
Like my grass-cutting comrade, i'm not one who is usually bothered by grammatical errors. Unless someone mixes up 'been' and 'being'. That really annoys me.