Christmas trip for large family

[bigdisneygroup]

We just booked a Christmas trip for 15 of us and already the scheduling fun is beginning. My question is this- we have 3 rooms at CB from 12/21 to 1/1. My one DD is now saying that she probably won't be able to join us there until 12/26. How do we handle this? We have a package with the tickets and dining plan included - can she just come down later and join us or do we have to somehow make her a different reservation? I am fine with just leaving her on the first ressie as we scored free dining but I wasn't sure how Disney handled it. Thanks for any info you can give.