Usually it's all or nothing: a place unionizes, everyone is a member of the union. Technically, you could say "no I'm not going to be a member". But you're still part of what is called the bargaining unit. In some cases it is even possible to opt out of the union dues... there was a Supreme Court case about it recently that came down on the side of allowing opt-out, but I don't know the full extent/ramifications of that ruling.