It’s not your job description that determines it, if it is designated a union shop you have no choice but to join the union.
Generally only a few support staff like secretaries and production assistants as well as management are exempt from being union members. If you are a temporary worker they usually have a union permit form that you fill out and they give you temporary or daily status. I had a job that they gave me union status on a day to day basis when I worked at the union shop. They deducted a small union fee from my check. After a certain amount of days worked they waived the fee for the rest of the year.
Getting in the animators union is a good thing. It means $ per frame for you rather than pennies.