I don't think the Amazons have the level of governmental organization that we do. Odds are, Donna and Derinoe were able to pull aside those they knew would commit to their plan and have them keep it on the "hush hush" from the others that wouldn't. If they were planning such a thing, they were probably as quiet about it as possible and I believe the attack happened at night when most of the other Amazons were sleeping or getting ready to retire for the night.