Definitely still here for this.  Awaiting the OOC.  

You could just allow anyone to join(who has an acceptable character), but have strict rules about disappearing players.  Say, for instance, if the player is incommunicado after X days, the character dies in the next flight.  Warn them through PM that this is about to happen, and if they don't intervene the character is gone for good.  This would a) deal with MIA RPers, and b) give us some character deaths, which really drives the war home for the remaining players and characters alike.