you can add the dining plan 72 hours before check in. I have heard they charge a fee for adding it with in the 45 day window though. I think it would still be a good idea I mean you could always use your TS for a breakfast together then all go your different ways during the day. Your family liked it before so I think it would work for you again. Even if some go to a dinner somewhere and some go some where else. I guess I would talk it over with the other members of your party and I know it might be hard but come up with a small plan of who will do what and then decide if then it is worth it for all of you to get the plan.
Whatever you decide I hope you have a very magical vacation!!
I had three room only reservations for this trip......one I converted to
free dining (Disney VISA code) about 2 months before we arrived and of course the balance was due 45 days out. The other resort stays weren't in the free dining dates, so I left them as room only until 5 days before each started and then called and added the dining plan. I did this for two reasons...first I wanted to be sure that we enjoyed the dining plan before committing.....but also because I didn't want to pay 45 days out and have Disney have my money already, lol.....and just in case something happened and we had to make changes or cancel, I wanted to be very very close to our dates before committing to the dining plan.
Neither time that I added dining 5 days before arrival was I charged any extra fee.
I would sit your group down and try to make them understand how the dining plan works. If they aren't willing to plan ahead, it could prove to be an expensive option for them if they aren't able to get reservations on short notice or walk up. Most places we were at were turning folks without reservations away....not just ...we'll see if we can fit you in....or it might be a bit of a wait....but outright saying we are all booked for tonight, and giving out the ADR phone number suggesting they make a reservation for a different night. The places I saw walk ups happening were the less popular resort restaurants...Boatwrights, Cape May, and surprisingly Tokyo Dining. If your party is willing to call around (or have concierge do so) and take whatever they can get each night, then it would work for them....but if they are the type that would say we're at this park/resort so we're eating here or we'll just grab a CS.....it could be quite an expensive plan.
I found that most people don't understand the process and think they can walk up and just have a bit of a wait but still eat......once they realize that doesn't work at WDW very well (and probably even less so than I'm seeing if you're travelling at a busy time!).....they come around to seeing the value of planning their TS meals. If not, then the dining plan may not be right for them.
Remember, though, that the dining plan is based on those in the same room.....just because you're travelling together doesn't mean that all 11 of you have to be on the dining plan. Your family can get it for your room....and let each other room decide for themselves. You can all still eat together if you choose......you just let the server know that your family's meals will be covered by the plan, and the rest won't. I saw that happen quite often and we actually did it once when we met some friends unexpectedly in the waiting area and had our tables combined....we had the plan, they didn't, and the server actually used our plan to pay for the 3 most expensive entree/dessert/drink combos at the table, instead of what WE specifically ate. I seriously doubt she was supposed to do it that way, so don't expect that of course.
Remember too, that there are a lot of threads on DIS about the frustrations of planning your vacation with other family members who aren't planners or don't "get" the process. So work hard to keep yourself out of that situation by thoughtfully and carefully considering if you really want to be involved in planning someone else's vacation!! I'd love to go with a group that large....but I think I'd plan my own little family's schedule and offer it up to the rest for their wanting to join us....but I definitely wouldn't want to schedule their days.....only offer advice based on my past experiences with being here. It would be hard for me as a type A planner, but from the threads I've read I think I'd be much happier offering advice and shrugging if they choose to ignore me, lol.
I hope your trip is magical and special for all of you!