It is possible to maintain agro on all targets even if DPS target different ones and go all out, but it requires you to keep using AOE skills constantly. Which would drain TP really fast.
Personally, I don't care too much about DPS in different targets. I usually open with 2 Overpowers, sometimes even Infuriate/Steel Cyclone followed by Overpower, and change to the single target rotation. If I realize something is slipping, I simply change my target to it and continue rotation, usually its enough to get agro back. At 5 stacks, Steel Cyclone if more than one add is alive. If you feel confident about maintaing agro, you switch to Deliverance, and melt things down!
Oh, and quality of life tip... Don't bitch about people hitting different targets if everyone is quiet and silent. Just do your best to keep agro, if something slips you go after it, get agro back, if DPS dies its their fault for not respecting the unspoken rule of hitting tank target, everyone is happy. If you start bitching, and since there ARE many tanks that can hold agro pretty well, you will get flamed, specially if the DPS happen to be elitists... Which, if they are stealing your agro, is probably the case.