C.S.
If your friends are flying in to Oakland Airport, I highly recommend Fox Car Rental. Their prices are much less than all the others, they do have occasional promotions and all-week deals, and there is a shuttle from the terminal.
If they use Fox, ask for a Doge Caliber. They'll try to give you a PT Cruiser in that class, and that car will NEVER make it up the hills to Yosemite. The Caliber was impressive - got me over the Grapevine in the fast lane - and had better gas mileage than anything else its size.