Lesson 258: いっしょに: Together with
The word 一緒 means 'together'. When it is used with the particle に, it expresses the idea of doing something with someone. It can be used to invite someone to do something with you, for example: 一緒にレストランに行けませんか (Let's go to the restaurant together?).
When preceded by the particle と, it shows who you are with or who you are doing something with. So for example you can say 来週、兄と一緒にハワイに行く (I'm going to Hawaii with my brother next week).
A common situation you might hear 一緒に in Japan is at a restaurant. The server might ask you how you would like to pay the bill (お会計), and ask if you will be paying together or separately:
「お会計はご一緒ですか？」Will you be paying the bill together? (Note the formal use of ご before 一緒).
To which you could answer either: 「一緒にお願いします」or「一緒でお願いします」(We'll pay together). Here and in the below examples, you can use either に or で without changing the meaning. We will cover this use で further in a later lesson.
Or if you want to split the bill, you would instead say: 「別々にお願いします」or「別々でお願いします」(We'll pay separately).