I'll send out probably 5-6 more PMs today. The leftovers could at least respond the the message saying they won't participate. Blatantly ignoring them is disrihspectful and just plain rude. It would take ten seconds to type "I can't send."
They're all new members, as well. Not making the class of 2014 look good.
They've all been online recently and I really doubt they don't have ten seconds to reply. If their day is that maxed out then maybe they shouldn't have signed up for an event.