Mensa International has a Constitution which sets out the purpose and infrastructure of Mensa. There are three aims:
- to identify and foster human intelligence for the benefit of humanity;
- to encourage research into the nature, characteristics, and uses of intelligence; and
- to provide a stimulating intellectual and social environment for its members.
Each country has its own national Mensa constitution, which is based on Mensa's Minimum Constitutional Requirements, the document that lays out the basic requirements for all national Mensas, although national legal requirements take precedence, of course. The Mensa Constitution is the over-arching document that governs all national Mensa groups.
You can view the Mensa Constitution here.