Honestly, I can easily see how this would be severely abused; and personally, I detest the idea. Anger ONE player in any way, and he or she will set your party to enemies and raid your base while you sleep. Somebody could get bored and decide to set enemies with every single party he or she can find, and destroy all of them. Maybe if it was contained (like both parties had to accept in order to set enemies, and you could only be enemied with one party and its allies), it would be plausible, but I think there's a lot more to consider, personally.
Have you posted in the suggestions tab? I'd recommend putting this there.
Also, HI Manyrain! Nice to see you here.