You can follow the constitutional provision for this purpose. The Constitution of India provides for freedom to assemble and the freedom to associate. Article 19 (1)(b) provides that all citizens shall have the right to assemble peaceably and without arms and Article 19 (1)(c) accords all citizens the right to form associations or unions or cooperative societies.
The policy can be stated that "All employees shall have the right to form or join any association of employees and there shall be no discrimation against an employee on this account. The right shall be regulated by the statutory provisions and care shall be taken to protect and promote the image of the company as a corporate citizen".