Actually, if you select a allied ship, structure or system, you have two pings, ally attack-ally defend. You click the one you want, then click the system you want targeted, and the AI will respond, it may just take some time because they seem to bring ALL their ships. It's great for weakening pirates or diverting another AI's forces away from the front where you plan to hit.
End of FearlessDefiance's quote
I know this already, but is knowing which planet to select as my ally source. I assume that clicking on a planet that has no forced in that particular planet is still going to amass a force. I have three allies. I need all THREE!!!