Meet yourself for a moment. Say you support a football club (the Reds), you are Brazilian, you are an engineer, and you work at a firm called Acme. Now walk into different rooms and watch which one of those "you"s switches on.
You carry all of these at once: Reds supporter · Brazilian · engineer · Acme employee. Only one tends to feel live at a time.
Step into a room of…
Same person, different rooms, a different answer to "who am I here?" each time. You never changed. The room did. A category switches on when the differences inside it look small next to the differences to whoever else is present, so the company around you, not you, picks the "we" (Turner et al., 1987).
Look again at the room of non-fans. There, Reds and Blues merge into one "football fans," a whole level up. Your identities nest, and the context chooses the level. Once a "we" is live, you take its norms as your own and act for the group.
The practical move: do not argue with one fixed self. Make the right shared identity salient, and get the category right, because the same message means different things depending on which "we" is switched on. One honest note: the theory explains which identity fired far better than it predicts it, so in a real, messy room, test which "we" actually switches on rather than assuming.