This is a very important question and i will try to answer it the best way that i can.
Lets say you're attacking a target with 100 crystals, you send a squad and your alliance member sends 10 squads, the crystals will be split according to how much damage both your units did and how many surviving units you have. In this case if you both have good squad build up then you will most probably get barely 8-10 crystals and not more and there's a chance that your alliance member will also get the conquer due to the large difference between your forces.
For more clarification you can go to our wiki page, click this link:
http://wiki.battledawn.com/index.php/Battle_System