I have been told by many, that grizzly's won't attack a group of 4 or more and that such an attack has never happened. That this alone is the best defense against attack. Here were 7 people.
According to the
article Colorama linked to above, the attack began when the lead guy was up ahead and the others were still crossing the river ... specifically it said:
"The first hiker had apparently made it out of the water and was out of sight of the others when he began screaming, Palmer said."Based on this and the earlier suggestion that the river could have masked the hiker's noise, it seems very possible the bear only became aware of the other hikers after the attack had begun.