CCP didn't really elaborate enough for me to be sure that's the case - in order to bring in allies, a defender has to make the war mutual.
But in the current system, making the war mutual removes all bills, doesn't it?
So what happens in the new system? If there are no bills to pay, how do you "abandon" it in that fashion?
A defender doesn't need to make the war mutual, that's completely separate. Making it mutual just means it doesn't cost anything to maintain, although the attacker still has to pay for the first week.
The defender can put out a allies contract (it doesn't have to be for ISK, if the friends want to war for free) and once the allies accept it, they're in the war as full participants (after 24 hour warm up), except that they are tied to the fortunes of the defender. If the defender surrenders, the allies war is over. If the war gets extended, it's extended for the allies as well.
I *think* that if a war is mutual, one side would have to put up a terms offer and the other side would have to accept it, or put up a counter offer ad nauseum until someone accepted it.
I seem to recall, though I would have to check my notes, that the defender has to make it mutual in the first 24 hours, I.e. before it goes live.