Employees shall have the right of self-organization and the right to form, join, or assist labor organizations, to bargain collectively through representatives of their own choosing, and to engage in lawful, concerted activities for the purpose of collective bargaining or other mutual aid or protection, and such employees shall also have the right to refrain from any and all such activities, provided that employees may be required to join a union under an all-union agreement as provided in section 377-6(3).