"All Germans shall have the right to form corporations and other associations. -- Associations whose aims or activities contravene the criminal laws, or that are directed against the constitutional order or the concept of international understanding, shall be prohibited."
This article says that all German citizens can found associations and societies such as private limited companies. It can pursuit a good cause (like protecting bees or helping the homeless)-- or a profit-oriented company. Individuals can form groups and fight together for common goals. However, it is forbidden to found an association which violates the law. Establishing a club of right-wing extremists or terrorists is therefore prohibited.
Article 9 also says that workers can form unions which fight for better working conditions. Similarly, employers can set up their own associations, i.e. employers' associations ("Arbeitgeberverbände").
Article 9 pertains a civil right. So it only applies to German citizens. Nevertheless, non-Germans can found associations in Germany, as the European Convention on Human Rights, the UN Convention on Human Rights and the law on associations acknowledge them the right.