In Lesson 4 of Genki 1, it states that one of the meanings of と is "together with". e.g. メアリーさんはスーさんと韓国に行きます。
However, the adverb of "together" is 一緒に and when と is added it has a nuance of "taking action together with someone". e.g. 私は彼と一緒に学校にいきます。
What is the difference between using just "と" and using "と一緒に"?