As I think I mentioned in another thread, I am happy to have anyone who wants to join up. I can't give you an exact member count at this point, but I know that we have seen a good amount of interest in a Sith Empire guild given that I haven't posted on the TOR forums about it yet.
As for the leadership structure, I am currently the only leader and we have no officers, but I don't intend to keep it that way. Eventually I plan on having multiple leaders (always an odd number, if possible). I would probably go with either three leaders or possibly five if the guild gets very large, with a complement of officers to cover whatever needs we have in that area. I'm not really concerned about making those changes right now, since we don't even know when the game will be released, but I will do what I can to make sure we are ready at release.