Why is みんな sometimes used with a particle and without?


I've seen in Genki I みんな used without a particle (no particle follows the
word). For example:

> たいていみんな寝ています
>
> 私のかぞくは四人です。みんなとてもしんせつです。

However, I've also seen sentences in which it is followed by a particle:

> あなたがみんな **を** 救った
>
> みんな **が** 日本に興味をもっている

Using it with a particle makes sense to me, since I translate it as everyone.
Why is it sometimes followed by a particle and sometimes not?




From the chat, by @Chocolate:

> たいていみんな寝ています -- here みんな is an adverb. the subject is omitted  
>  "They are all asleep"
>
> みんなとてもしんせつです。 here again みんな is an adverb. No particle required. The subject
> is 家族 "They are all kind."

Another example:

> みんなが出かけたあと、～～ -- After everyone left, ~~
>
> here みんな is a noun
>
> 家族がみんな出かけたあと、～～ -- After my family all left (or, everyone of my family left)
>
> here みんな is an adverb

A few more similar words (which work both as nouns and adverbs):

  * 全部
  * 全員
  * すべて

