Aid Another

[Xanatheus]

Can an invisible ally "Aid Another" to grant a visible ally the +2 to attack or AC? Can an invisible creature grant the +2 bonus for flanking an opponent to an ally?

[ignimbrite]

I could see the +2 AC bonus, it is not really an offensive action so invisibility would not be banished. I see it as the bad guy takes a swing and the invisible ally tugs on bag guys coat or nudges the elbow or something and this imparts an effective -2 to attack, which is kinda the same as a +2 AC.
Aiding an attack seems to be a little bit more difficult but you could use the same tactics ... maybe ...?

[Farcaster]

I agree that they could aid and give an ally +2 attack or +2 AC. In the case of giving a +2 attack, they would become visible though. If memory serves, they have to make an attack roll versus AC 10 to even assist, so by definition they would become visible. As to flanking, they certainly could flank and would not become visible.

[cigamnogard]

What he said

[Xanatheus]

Assume the invisibility is greater and remains invisible after attacking, does an invisible creature that is flanking a mutual enemy grant the flanking bonus to an ally that is also flanking. If yes and since the invisible attacker may not have been pinpointed nor the actual square known, can he grant flanking to an ally that is not in the actual flanking square (for instance a rogue's sneak attack).

[rabkala]

The flanked opponent does not have to know he is flanked. It is a simple mechanic that did not get carried to extremes. Can an invisible foe get sneak attack on an unknowing victim? Why yes, same as he can provide a flanking bonus. Can an invisible foe aid another, Yes again! I think we have found a trend. Of course, in 4e you would explode...