@Sailor Vulcan: I'm not completely sure that I follow your last message, so in case you are confused let me try to clear things up. The only thing that matters is who controls the guarding creature. If the guard is friendly to me, then I ignore it. If the guard is an enemy, then my creature's melee attacks are limited to either attacking the guard, attacking itself, or attacking an object attached to itself.
Neutral objects are treated as enemies, and thus always apply these restrictions when guarding to all players.
So, if player 1 wants to attack a sleeping creature, they can ignore any guards controlled by player 1 because the guards are friendly. But, if there are any guards controlled by player 2, an enemy, then they would have their targeting options restricted, preventing them from attacking the sleeping creature. Regardless of who controls the sleeping creature.
Now, in a FFA game, all other players are considered enemies. There are no rules that allow for a temporary ally to be treated as friendly, unless you house rule it. This means that if Player 1 puts a creature on guard to protect Player 2's creatures, then it not only restricts Player 3 (accomplishing the goal) but also restricts Player 2. Which may not be so desirable. This is similar to how my creatures will continue to hinder any temporary allies I may have.
Interestingly, this conversation came up while playtesting the Forcemaster Academy set, as there is an enchantment, called Mind Seize, that lets you choose the target of a creature's attack. Thus, letting you make your enemy's creature attack its friends. This will actually let you bypass your enemy's guards, because they will be friendly to the creature making the attack. However, weirdly, if you have any of your own creatures on guard, they WILL still restrict the creature's attacks, because they are still enemies of the attacker. But, in this case, you can always just make the creature hit itself.
