You must not have a lot of AoE in your group. So long as your group does 30% damage to targets (Which with a bunch of AoE it shouldn't be a problem.) You'll get credit for the kill. It doesn't matter who tags the mobs first. I've literally gotten up to chain 103 in a Dark Devices group. We usually had 1-2 Black Mages, 2 Dragoons, and a Whitemage in our set up.