This would then apply to duelling as well. And it would add something else, too: when you are grouped with someone, and that someone is either involved in a guild war or accepted a duel, then th entire group would become attackable by the opponents the one player agreed to.
Things that must be worked out are: what happens when
1) the player is in the group but wasn\'t attacked or didn\'t attack, and quits the group? IMO, they should not be attackable afterwards
2) the player either was attacked or did attack and quits the group: IMO, they should then remain attackable
3) the group is disbanded by the leader: does 1) or 2) on a per case basis with all involved players independantly.