How early must I arrive to get seats to Shakespeare theatre company's free for all?

I am looking to go to the Shakespeare Theater Companies "Free For All" to watch Julius Caesar with some friends. The Only time we can go is opening night, which will be a Thursday at 8pm. If anyone has ever gone to one of the Free For All events before and can tell me when I must show up to the theater to be able to get tickets to the show, or anyone who might have some general idea, I would greatly appreciate the help.