My grandma is taking us to the zoo, she isnt on the membership card but me and my brothers are, can we still use the card or do we have to pay(as in me and my brothers i know she will have to pay)?

You would have to be more specific about your question. Pay for parking, pay for access et? This is a question that you should call member services about etc.

You two will be free if you two are on the card

if you have the card your grandma can buy all the tickets

It depends on which membership plan you took one in is just family and the their is one that includes a guest and the membership card will tell you if you can bring a guest.

I understand it isnt transferable, i am on one but im under 18, can i use it without any of the adults on the membership

Membership isn't transferable.