Your grandfather would need to establish an account as well with USAA. I'd be a little surprised if he didn't have one already for insurance.
Military and veterans and their spouses. Also, children of members. So you would need grandfather or grandmother to join. Your parents to join then you could.
At the very least, the grandparent purchasing property and casualty insurance would be involved as well. One of your parents might have to as well. "Full membership" that can be passed down is obtained by purchasing property and casualty insurance. "Insurance-eligible" membership allows you to have full banking services, but you cannot pass it down to the next generation without buying P&C insurance, I believe.
USAA is not a credit union.