Well, here's my experience in April for a party of 12. I called regular dining to see which ones they could do (vs. being transferred to group dining), and they said they could do all. So I started going through my choices in order of importance, and out of 4 I could only get 1 first choice, and 1 second choice! So later I went online and starting looking at smaller groups, and I was able to get what I wanted by splitting the families up (7 and 5). I figure we will just ask to be seated near each other, but if it doesn't happen, no worries.
I have 2 reservation numbers because we have 2 villas, so I booked one family under one and the other family under the other, so that Disney would not think I had booked it twice in error or something and delete one of them. That's also why I didn't split it in half 6 and 6 - I wanted the reservations to look different in every way. But I got approx the same times for both, and we will arrive together and ask to be seated near each other if possible. Or, we might ask in advance at the actual restaurant e.g. we have breakfast at Tusker one day, but we're going back there for lunch - we will tell them at breakfast that we're coming back for lunch, and that we want to be together so that they know how to set up the tables later. If you're eating in park sometime after arrival (i.e. late breakfast, lunch or dinner), it might be worth doing the same. Or, if you can get the actual restaurant number, maybe you could call in advance.
