Looks like it is possible for 30 people to gather without getting killed in DayZ after all. It was one of those rare occasions, organized by the DayZ Flashmob group on Steam, and they were lucky indeed that no one brought an M4 or grenades to the party.
And you might wonder what they did after the meetup. Did they go out to kill zombies? Absolutely not:
Things to do with 30 people in DayZ [YouTube]