SF2G is an easygoing, bicycle-commuting, club-like entity with no real structure. Ok it's more like a mailing list. Its purpose is to make it fun & practical to ride your bike to work from San Francisco to points down the peninsula, often as far as Google, Apple, Facebook, nVidia, etc. Commutes are typically 20-50 miles. Riding that far in the morning is a lot more enjoyable with a posse, so SF2G was created to facilitate that.
There are several routes and several ride styles (speeds, essentially.) The routes offer a wide variety of terrain, including nearly completely flat, to hilly and challenging. Ride styles range from casual and friendly to blistering. The entire range of combinations is covered throughout the week. You pick the ride that sounds like your cup of tea.
Interested? Then just join the Google Group (mailing list) and observe. Pretty much every day someone will announce a ride leaving from somewhere in SF, usually a coffee place such as Philz or Ritual Roasters (both of which allow you to bring your bike inside), along with info on what route s/he has in mind, the pace, etc. The ride "calls" will usually have some acronyms and jargon, but they aren't too cryptic.
What about getting home? Well, if you don't have a corporate shuttle, either ride home (people do -- watch the list) or take Caltrain. On friday, take the Partycar!