I like the energy but giving free tickets to people that already have season tickets or to fraternities and sororities is largely preaching to the choir. These are the groups that are already going and the efforts will likely only increase attendance marginally.
The free tickets need to be given to those who wouldn't be going otherwise, to get them interested and create new fans. Most important is freshmen, as they have only been in school a month and are looking for social interaction, new activities, some will be pledging the Greek system, but someone who knows can tell me if they would be in yet, I think not and certainly they don't live in the houses. The dorms is where you will find the most freshmen.
The other large group to offer free tickets to is potential unaffiliated fans, but I'd start with East Bay and SF students, college and HS. Online offer, General Admission, student section and end zone, just bring a student ID, admit until those sections are full. If you are late, find Tightwad.