The reason why the tank fills up with the entire group is that, if they take the first action against the group, all the other enemies that come with it make him their primary target, and the hotbar has them show up to say "This enemy is targeting you" The reason you have to hit them individually is because when you're not the tank, the monsters do not care about you at the start. If the tank were to die (and you weren't right in front of the mob), they wouldn't go after you but rather head back to where they were pulled. You need to get at least some enmity on the mob in order for it to care about you, then it will show up.

On a side note, an easy way to target enemies is to use the F-keys to target the tank (So, if he's in the second slot that'd be F2,) then press "T" to target the tank's target, which should be the enemy being focused on.