One of the most important rules of PvP - dislike before mechanical fighting!
This is usually easy, especially with the hostilizer... unless you're joining an ongoing fight between two sides. At that point it's too hectic to get a party invite so you can '!' everyone, and if you do '!' everyone, you're likely to get battered by your allies by mistake.
I would like to suggest a function similar to hostilizer, that dislikes everyone that the target has set to dislike themselves already. This way, if a fight is ongoing, you can click a pal you know is on the 'right' side, and join in the fray with the same enemies.
The limitations of this of course being you can still be disliked by them, but this might reduce mechanically caused friendly fire.