There is no way to know this without knowing a great deal about where you are and what employer in particular you are talking about.
First of all, there are states which have right-to-work laws. These are laws that explicitly state that it is illegal to have a work place in which employees are required to joing a union. If you are in such a state, then employees in any unionized workplace have the choice to join or not.
Within states without right-to-work laws, there are different sorts of workplaces. In some, union contracts mandate that all workers must join the union. In others, it is optional.
Therefore, there is no way to give an answer that covers all employers in all states of the US.
i find no options for labor for not joining labour union. labor union is the ultimate and last resort to settle and satisfy the agreements with the management and for establishing a fair work practice in the organization. (but still sometimes the union may go on biased form). as far as concerned with options for joining labour union, its better the labor should be intact with the norms of union and for welfare of labour.