All depends on whether the person who's already at that table wants you to sit with them.

Is this an underage question? Either way, if you are over 21, you can sit wherever you want. If you are under 21, you can sit wherever you want as well. As long as you don't have an alcoholic drink in your hand and aren't consuming it, you are fine. What the other person at your table is doing is not on you. Ever go out to eat with your parents and one of them ordered a beer/margarita? Same thing.