In my mind, for soaring nearest metro bay area it's a tossup between
NCSA at Byron and Hollister Gliding Club at Hollister.

NCSA is about an hour east of both San Fran and Santa Clara. It's a
club, and I know that they do instruction, but I can't remember if
they rent gliders:

http://norcalsoaring.org/
http://ncsa-buzzard.blogspot.com/

Hollister is about an hour south of Santa Clara and an hour forty
south of San Fran:

http://www.soarhollister.com/

Other good but more distant options are Avenal (south), Williams
(north), Montague (way north), Truckee and Minden (both east),
California City and Tehachapi (both way south).

It's a pity you have to drive so far these days. It used to be that
the Bay Area was ringed with gliderports now gone: Sky Sailing in
Fremont, Lagoon Valley in Vacaville, Hummingbird Haven in Livermore,
and also the gliderport at Calistoga. Eh, that's progress for you.

Give me a shout if your conference is near Sunnyvale, let's talk about
soaring in China!

Thanks, Bob K.