To get to know each other; to make someone's acquaintance — to meet and become acquainted.
友達の紹介で彼女と知り合った。
I met her through a friend's introduction.
大学で多くの人と知り合うことができた。
I was able to meet many people at university.
ネットで知り合った人と会うときは注意が必要だ。
You need to be careful when meeting someone you met online.