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

Question

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?

Answer 1

Not only can particles be omitted sometimes, especially in colloquial speech, みんな can also be an adverb as well as a noun.

For example:

私の家族はみんな音楽が得意です。

Everyone in my family is good with music.

人はみんな違う。

All people are different.

Answer 2

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