It would be nice if similar to Age of Wonders that if you get attacked and your teammate or alliance member has units nearby then they can load into the battle to to help..

Instead of having a 3rd person being involved in combat, an ally with troops in the battle area could be given a prompt to "loan" out troops to be controlled by their ally. This could get around the feature that only 2 sides can be involved in combat, but sometimes your allies don't have the best intentions for loaned troops.

I just like in a multiplayer game that me and my friends can be in the same combat to help each other in planetfall.. would be awesome for this one too if its possible

I don't see why Humankind can't just allow allies to reinforce their own armies into battle.