I like it. Tarner has a pretty good Endurance score, so I guess he'll be the one to hold the shield. Yeah, he'll probably lose his Unarmored Agility bonus, but the shield will bring it up +2, thus cancelling it out. Plus, if he moves 3 squares or more on his turn, he gains concealment. He could also use Ethereal Stride, and so even with the shield it'd be pretty hard for any of them to hit him while he protects his friends.
EDIT: Oh hey, the shield stacks with Unarmored Agility. Combined with Ethereal Stride, this will bring Tarner up to 20 AC, plus partial cover plus concealment.
ANOTHER EDIT: Okay, so Gruunk can also take a shield, and thus the entire party will be protected. He's got better Endurance than Tarner, not to mention higher AC, although Tarner can come pretty close.