Employees have the right to unionize, to join together to advance their interests as employees, and to refrain from such activity. It is unlawful for an employer to interfere with, restrain, or coerce employees in the exercise of their rights. For example, employers may not respond to a union organizing drive by threatening, interrogating, or spying on pro-union employees, or by promising benefits if they forget about the union.

Section 7 of the National Labor Relations Act (the Act) guarantees employees “the right to self-organization, to form, join, or assist labor organizations, to bargain collectively through representatives of their own choosing, and to engage in other concerted activities for the purpose of collective bargaining or other mutual aid or protection,” as well as the right “to refrain from any or all such activities.”

The law says you have the right to join a UNION

Section 8(a)(1) of the Act makes it an unfair labor practice for an employer “to interfere with, restrain, or coerce employees in the exercise of the rights guaranteed in Section 7” of the Act. For example, you may not

• Threaten employees with adverse consequences, such as closing the workplace, loss of benefits, or more onerous working conditions, if they support a union, engage in union activity, or select a union to represent them.
• Threaten employees with adverse consequences if they engage in protected, concerted activity. (Activity is “concerted” if it is engaged in with or on the authority of other employees, not solely by and on behalf of the employee himself. It includes circumstances where a single employee seeks to initiate, induce, or prepare for group action, as well as where an employee brings a group complaint to the attention of management. Activity is “protected” if it concerns employees’ interests as employees. An employee engaged in otherwise protected, concerted activity may lose the Act’s protection through misconduct.)
• Promise employees benefits if they reject the union.
• Imply a promise of benefits by soliciting grievances from employees during a union organizing campaign. (However, if you regularly solicited employee grievances before the campaign began, you may continue that practice unchanged.)
• Confer benefits on employees during a union organizing campaign to induce employees to vote against the union.
• Withhold changes in wages or benefits during a union organizing campaign that would have been made had the union not been on the scene, unless you make clear to employees that the change will occur whether or not they select the union, and that your sole purpose in postponing the change is to avoid any appearance of trying to influence the outcome of the election.
• Coercively question employees about their own or coworkers’ union activities or sympathies. (Whether questioning is coercive and therefore unlawful depends on the relevant circumstances, including who asks the questions, where, and how; what information is sought; whether the questioned employee is an open and active union supporter; and whether the questioning occurs in a context of other unfair labor practices.)
• Prohibit employees from talking about the union during working time, if you permit them to talk about other non-work-related subjects.
• Poll your employees to determine the extent of their support for a union, unless you comply with certain safeguards. You must not have engaged in unfair labor practices or otherwise created a coercive atmosphere. In addition, you must (1) communicate to employees that the purpose of the poll is to determine whether the union enjoys majority support (and that must, in truth, be your purpose); (2) give employees assurances against reprisal; and (3) conduct the poll by secret ballot.
• Spy on employees’ union activities. (“Spying” means doing something out of the ordinary to observe the activity. Seeing open union activity in workplace areas frequented by supervisors is not “spying.”)
• Create the impression that you are spying on employees’ union activities.
• Photograph or videotape employees engaged in peaceful union or other protected activities.
• Solicit individual employees to appear in a campaign video.
• Promulgate, maintain, or enforce work rules that reasonably tend to inhibit employees from exercising their rights under the Act.
• Deny off-duty employees access to outside nonworking areas of your property, unless business reasons justify it.
• Prohibit employees from wearing union buttons, t-shirts, and other union insignia unless special circumstances warrant.
• Convey the message that selecting a union would be futile.
• Discipline or discharge a union-represented employee for refusing to submit, without a representative, to an investigatory interview the employee reasonably believes may result in discipline.
• Interview employees to prepare your defense in an unfair labor practice case, unless you provide certain assurances. You must communicate to the employee the purpose of the questioning, assure him against reprisals, and obtain his voluntary participation. Questioning must occur in a context free from employer hostility to union organization and must not itself be coercive. And questioning must not go beyond what is needful to achieve its legitimate purpose. That is, you may not pry into other union matters, elicit information concerning the employee’s subjective state of mind, or otherwise interfere with employee rights under the Act.
• Initiate, solicit employees to sign, or lend more than minimal support to or approval of a decertification or union-disaffection petition.
• Discharge, constructively discharge, suspend, layoff, fail to recall from layoff, demote, discipline, or take any other adverse action against employees because of their protected, concerted activities.

NLRB Basic Guide to the National Labor Relations Act