Association is a form of permanent organization, in which a group of people or institutions seek to perform certain common interests. In other words association is an organized group permanently.
Joining is a right for all. The condition is to respect the laws as all citizens.
Go to OAB (Brazilian Lawyers Bar Association) in the state where you are. There is a public service for free to help people who want to form an association and legalize it.
It is important to know that the association is under protection and obligation of state law.