Anyone, onsite-offsite-local, can book dinner 180 days before the desired dining day. Since you are well into that timeframe you should check online multiple times a day to see what is available. Most likely everything is booked and you will be hoping a cancellation enters the reservation system just as you are checking. Everyone who might be at the table needs to be included in the reservation, but there aren't many tables for that large a group. If you are expecting 12 people you can look for 2 or more smaller reservations as close together as possible in time. You can request to be seated together when you check in at the podium. The CMs will try their best, but there is no guarantee. Most likely late evening times will be your best chance for getting reservations.
If being seated together or near each other is important then lunch might be a better option because you can choose your own tables. You'll have to wait in line since FP+ is not available for offsite guests. Many have posted that while the line looks long, it often moves faster than expected.
