Ahoy Stephanie and =welcome to Cruise @ddicts =sailor
Group rates require a minimum of eight cabins with sixteen passengers so it sounds like your group would qualify.
There is no set standard for group discounts. It just depends on the ship and sailing date. Sometimes there is no discount at all and other times there is a very good discount. Group rates are not combinable with other promotions such as past guest, seniors, etc. When booking as a group you will normally find a lower deposit than individual bookings and more flexibility with changes, etc. depending on how far in advance you book. You will also usually be offered "amenity points". Again, how many points depends on the ship and sailing date. Points are used to aquire perks for your group. You can get on board credit, a bottle of wine per cabin, bathrobes, beach towels, photo certificates, specialty restaurant, your choice of amenities and how many you get is based on how many points are offered. Each cabin will get the same amenity so no mix and match.
When booking a group you should have a group leader who would be the main contact with the travel agent. Also, pick a travel agency that will allow only one agent to handle your group from beginning to end.
I worked in a cruise groups department for two years and booked many family reunions. It is a great way for a family to get together.