IMO...I think it's fine if it's 5v1 for the 4 people to let their teammate duel the last remaining guy (I don't like it, I think you should gang up on them, but I think it should be allowed). But getting in the way of your teammates ability to hit the enemy is griefing. Get away from the battle. I don't engage every enemy I see, and I don't consider myself one to bro-code.
It's certainly a fine line, but at the end of the day, it's pretty obvious if someone is bro-coding or not. Play the game and you won't ever have to worry about if you're bro-coding or not.