Admittance to each event is subject to availability of seats, and is on a first come/first served basis. There are approximately 236 seats in the theater for all screenings except Battlestar Galactica, for which there are 800 seats. If a seat is available to you, you will be contacted via email. When all seats are filled, no further invitees will be admitted. Your RSVP via this form is not a guarantee that you will be admitted. No "rainchecks" will be issued for a future event if any event is full.
I wonder if the need for a bigger theatre might be a hint to the Emmy nominating committee?
BSG is the only series that doesn't have confirmed guests yet, but they are also the only series, I believe, currently in production. Considering Jamie lives in LA, I'm thinking there is a good chance he'll be there.