This might have been brought up before, but via search, I didn't find anything similar.
I don't really like the fact that 'you must be grouped with X to attack Y'. I know it has been created to prevent kill stealing, but it's somewhat unlogical. In my opinion, the best would be, if you could request your assitance, by attacking the mob, someone else is already fighting, without the need to group up with the person.
Example:
You see someone fighting an Ulber and want to help him. You target the Ulber and hit your attack shortcut. But instead of getting the denial message, your attack try should let a confirmation box pop up for the person you want to assist. Of course, a pop up is often quite annoying, especially in the heat of battle, and requesting a group has about the same effect. But, if you had the option to macro your confirmation, just like with the PvP request (always accept, ask each time, never accept), you could easily determine your attitude concerning the feature. I for one, would like to have the possibility of providing, aswell as receiving assistance and think, it could be an interesting feature.