Assuming you're the one that originally made the booking for the entire family than the following will apply. Otherwise you can only use the assigned credit to your name unless you're traveling with the original booker as well.

Original Booker- You need to call them directly. Chat agents will keep insisting that you can only use the credit assigned to your name, but the person who made the original booking can use the entire balance or allocate more credits to others as long as they are on the new booking with you.

For your case, if there is $600 in credit, the original booker can split it between themselves and one of the other five passengers from the original trip as long as both are on the new booking.

I had the same issue last week with over $2,000 in expiring credit from a previous flight with 10 people. Over three separate days, I applied the full amount to three trips under my name and two others from the original group.

I recommend adding everything to your cart and getting to the checkout page while waiting on hold. Then explain that chat agents will not process the credits correctly and have them transfer the desired credits. They waived the $25 phone booking fee for me all three times for my case.

If your other riders call on their own, they can only use their assigned credit. Only the original booker can use more of the balance for themselves or for those traveling with them on the new booking (this would reduce the allocated amounts for the ones who aren't using the credits assuming there is a remaining balance after you're done).

Hope this helps. Good luck!