I could be wrong (wouldn't be the first or the last time), but I think the term "invite" is a little misplaced. Go online to the link and sign up. Then you will get an "invite," as long as they don't run out of space. They need to do the "invite" so they can have the required info to be able to set up the event with people coming into the "secure" area of the airport. The MOC at SFO is on the secure side of things, so I am sure there are some fun requirements from the TSA of what all you need to provide before you can be allowed into that area (even with an escort)