What determines the rewards distribution in giant rally for each of the member who join?

Everyone in the rally (if it is successful) will get the core reward for defeating the giant.For their own first kill of the day. You can do more than 1 rally but don't get the rewards again.

There is also a gold reward for those who do damage to the giant. This will go to the person who started the rally and anyone else who does damage. Damage is done in order of joining the rally. For example if the person who started it does 50% damage then they get 50% of the gold reward. The second person perhaps does 40% damage, the third person may then get 10% and anyone else will not get any gold reward. If the first person does all the damage then no one else gets gold.