As a member of Mensa myself I do also have access to the online searchable member directory. All current members names are listed there. They may omit all other details (address, etc.) but I believe that their names would be there at least. No one with the first name Shakira is listed as a member. There are a very few Madonnas but I am quite certain based upon last names, ages, and addresses they they aren't the Madonna. There is also no Steven Hawking or Neal Pert.
Sorry I was not precise. By "many of these people" I meant to refer to top artists and intellectuals in general.
Here is one source. The * indicate "confirmed member of Mensa."
http://knol.google.com/k/iq-scores-of-famous-people-past-and-present#
