Maybe you're thinking of Geena Davis or Lisa Kudrow? They're in MENSA....

Youre right, but there was reason for him to think this. Says wikipedia:

Mensa controversy

For many years it was believed that Sharon Stone was a member of Mensa,[32] but in April 2002, she admitted she was not, and had never been, a member of the high-IQ society.[33] Jim Blackmore of Mensa said, "It's delightful to finally see Ms. Stone admit that she's not and never has been a member of our society. But then she goes on to say, 'I went to a Mensa school.' Not so."[33] Blackmore said that would not have been possible as there have been no Mensa schools since the early 1960s.[33]

Thanks for finding this clip. I've been reading about this gaffe for days now, as well as her efforts in damage control, and have also been counting on Sharon Stone for years now to cheer me up and lift me out of my usual state of cynical low-level depression with her occasional offerings of off-the-wall idiocy. I saw a clip of her at the AMFAR gala in Cannes a couple of years ago refer to 'countries like Africa'. I seem to remember an interview or something where she, referring to her fierce intelligence, said she graduated from college at something like age 19. The woman is just the latest upholder of a long and illustrious tradition of Hollywood looniness, and it just seems to get worse with age. Maybe it's that possibly exercise-induced stroke a few years ago. For more gems I recommend her interview in 'Searching for Debra Winger', where she says something about 'women being in their wonderfulness'