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"?
よろしくお願いします。
と is like "with", a particle to mark other parallel agent(s) involved in an action. That means:
メアリーさんはスーさんと韓国に行きます。
メアリーさんはスーさんと一緒に韓国に行きます。
They basically mean the same thing, but:
アメリカはソ連と戦います。 USA fights with USSR.
アメリカはソ連と一緒に戦います。 USA fights together with USSR.
～と戦う is just like "fight with", where you don't know what comes after is a friend or foe. Using と一緒に makes sure that it is in your party. (Note: Japanese has no equivalent of "fight against", so ～と戦う tends to mean the opponent.)
The same applies to ～と連絡を取る "make contact with" or ～と向かい合う "face with".
I agree with the explanation from Genki - と一緒に has an emphasis that you are doing an action together with someone. Having said that, I would say they are quite interchangeable.
