"Everyone is splendid, aren't they?"
You don't always need them. Like when you say "tomorrow, I'm going to the shops", you don't necessarily need a WA after tomorrow.
Whenever you are referring to a person or people a suffix, like さん、さま、くん、etc., is put at the end.
みなさん is the more formal version; it's actually sonkeigo (honorific speech) while みんな is the normal version.
^link to honorific speech explanation