If you research the ability to give AI players missions, I think you can try to give them an attack planet mission. There might be another way as well, but honestly I've never used it. Usually you just have to wait for the AI to do its own thing, or attack first yourself and hope the AI gets the idea.

I never use missions.

Get your allegiance with your allies high enough, click on one of their planets or ships, click the attack or defend ping button then select where you want them. If you did it correctly, you will get a message in the top middle of the screen telling you one way or another.

ID "IDS_TAUNT_TECH_AllyRequestAccept"
Value "Roger that!"

ID "IDS_TAUNT_TECH_AllyRequestAccept"
Value "I'll do what I can."

 or

ID "IDS_TAUNT_TECH_AllyRequestRejectionNotHappyEnough"
Value "What have you done for us lately?"

ID "IDS_TAUNT_TECH_AllyRequestRejectionNotHappyEnough"
Value "Your empire is in no position to make requests of us!"

These are a few examples of many, it will give you an idea of where you stand with them. It takes a few seconds for the response. You don't want to see any of the RejectQuestHate responses. 

If they tell you to go blow, you haven't earned it. Research a lot of the Diplomacy techs to help, get the give money one, it can help raise allegiance when you pay them off.

In Sacrifice of Angels 2, playing against multiple Borg on unfair, this is a MUST KNOW.

I found the ping button in general is only good for multiplayer teams, I could be wrong.