People join Mensa for many different reasons - for intellectual discussion on a diverse range of topics, for a lively social calendar, or to visit places or hear speakers often not available to the general public.
Some join just because they can!
To become a Mensan, all you need to do is demonstrate you have an IQ in the top two per cent. There is no other criterion. That means scoring at the 98th percentile or above on an accepted intelligence test. The test can be one conducted by the national Mensa in the country where you live or it can be an approved test that you have taken elsewhere.