Check in online at the same time and it should be fine. Even if your numbers aren't exactly one behind the other, you should still be in the same boarding group. As long as you can stand together when they line up for boarding you will be able to get on at the same time. It's general seating so your only problem will be finding six seats together.
However, if your plane is like ours was, the seating is 3 seats on each side of the aisle. It may be difficult to find an entire row empty unless you are in the very first group. You could also sit all on one side of the plane one set of three behind the other.
Try to check in right at the 24 hour mark and get in the A group and you should be okay even if you have to go further back to find all seats together.