Together
noun, の adjective
You are one and together. If you're together as one, you are together.
いっしょ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. The 一 becomes いっ.
「ご一緒してもよろしいですか？」「全然、構いませんよ。どうぞお座りください。」
"Do you mind if I join you?" "No, not at all. Please have a seat."