Hello adventurequeen, welcome to Cruise-Chat! You can sit together during dinner, however it would be a group of tables like a round table for 10 and a round table for 8. That can be arranged prior to your sail date through your TA - or calling the cruise line if you booked directly with them. I strongly suggest the you go to the dining room and find your tables when you board the ship. You will be able to make changes if you did to. There will be some dining staff assisting people.
Yes, you can reserve tables at specialty restaurant. Remember, there is an age restriction. Children under the age of 13 are not suppose to be in the specialty restaurant. When you book, you can only book for 3 couples - so, 3 people in your group will have to make the reservations. You'll need the booking number of each guest on the reservation. Again, there won't be a table large enough for 17 people. This was the information I got from the diamond plus desk this morning. You'll find that the cruise line will do their best to accommodate you. Let us know what you find out.