How do I send troops to an ally to help them fight?

If your ally starts a rally against an enemy then you will have a certain amount of time to join the rally, if you are too far away you cannot join. The amount of other players APC's that can join the fight will depend on the level of their Headquarters building. This is from 1 to 4, if the slots are already filled you cannot join.

You can also send troops as reinforcements to their city to help defend it when its being attacked. There is also a limit which depends on the level of their Embassy building. this will be from 1 to 4 APCs so if they already have reinforcements then you will not be able to send more.