maybe you just have trigger happy ARC/BRD in the group. Most 50 content encourages trigger happy behaviour and breeds bad habits but I certainly understand the frustration of trying to hold aggro regardless of trigger happy DPS or not. Tanks have very limited actions during low level and I found that the best way to get through is simply numbering adds. It gives me my prime target to spam attacks on and I usually don't have to many issues holding aggro so long as I'm allowed to get the first hit in. That being said, I've run BRD low level in 2.1 and found that if I do run a solid rotation off the bat, I will almost certainly pull aggro, but if I relax it a little and let the tank hit a couple of times before I start my full rotations, then there is no issues.
