To get involved with handcycling around the Bay Area, please visit the BORP Center in Berkeley's website here. They generally ride on Sunday mornings from March-November:
http://borp.org/cycling.php?textonly=

To visit the Official US Handcycling Federation's website, click here:
http://www.ushf.org/

To learn more about the different types of cycles, check out this article on handcycles:
http://www.centeronline.com/Feature%20Program%20.html