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?