I want to be the sole student author on a poster, but my advisor wants to bring on other students
Depending on how your lab works and how collaborative the work is, you may be right or wrong in this.
However, I suggest you keep a few things in mind. Fighting with your PI is seldom a way to academic success and having the good will of your supervisor probably has more value at this moment than having a poster with a small number of authors.
Also, the supervisor may, actually, be trying to get some of the others an interesting project with which they can boost their learning. Is it possible that she has done this sort of thing for you in the past? For others?
I'd suggest that you might just ask her for her reasoning in this, without appearing to be argumentative.
But you are probably (not necessarily) better off in yielding on this for the sake of your long term goals. Don't let short term goals interfere with the bigger - longer term - picture.