I believe EJ has been planning this for the past couple of episodes, even if it wasn’t stated, so he had plenty of time to run down to Kinkos and hire a few people to get them posted.
Since the flyers are all over the place, everyone had an opportunity to see them, so why did everyone act so surprised at his announcement.
True, I did not notice them at the very first, but since Bo and Hope were the host and hostess, I would have thought they would have noticed especially since their friend Abe was to make his own announcement at the party.