一緒に simply means 'together'. It describes doing an action with someone else or sharing an experience. You can say 一緒に勉強する (study together) or 一緒に食べる (eat together). If you want to say 'with whom', use と: 友達と一緒に (together with a friend). The と is optional when the companion is obvious from the situation.\n\nIn Japanese, 一緒に often appears right before the verb, but it can also come after the topic. For example, 私は一緒に映画を見ました (I watched a movie together) sounds natural, though usually you'd mention who you were with. It's a very common, everyday word used in both casual and polite speech.\n\nNote that 一緒 can also mean 'the same' in other contexts, but with に it almost always means 'together'.