It isn't possible to coordinate with the AI, but the AI may join you if you invade an enemy province. In such cases it's wise to see what kind of force the ally has brought along. If they have deployed a larger force then yours - they will win the province. If it's a smaller force you should win it - but you need to ensure that the ally does their share of the fighting. It's not good when your units do most of the fighting and dying and the AI ally wins the province.

You can also relieve sieges. If your ally is besieged by an enemy, you can invade that province to relieve the siege. This improves your king's influence as well.