My name is Suravi Chatterjee-Woolman and I have been a member for nearly a decade.
I took the test because life took me down a road better-not-travelled, and I decided to take the test as an intellectual telemetry of sorts- to find my way back.
Mensa to me it is like the Room of Requirement in Harry Potter. You can immerse yourself in genial pub meetings, or just attend days of knowledge with subject-matter experts, be a part of Special Interest Groups, or simply turn up at social events as often as hectic schedules permit. And just like in the world of Wizardry, "it is always equipped for the seeker's needs".
And finally I resisted taking the test for years. I have street smarts, I told folks and IQ tests can't measure that! In reality however I did not want to be a certified idiot. So just like the variety of reasons not to take the Mensa test, there are lots more for why people choose to be a member. Here's hoping misperception doesn't keep anyone from discovery!