You can email MONS and let them know you're planning on attending, or call 415-554-6110. The word on the street is that they may close the room off if it's full. (We hope this isn't a plan to keep Question Time advocates out -- Newsom would never do something like that, right?)
Update: The good folks at the Guardian have confirmed that Newsom's folks are sending out invitations to his supporters telling them to RSVP because "space is limited."Thanks to our tipsters for passing it along!