Hi, Katy!
It is so much fun to travel to Walt Disney World with another family. The more the merrier!
Each room will have its own reservation, guests, and Disney Dining Plan credits attached to that reservation. So your family will have their credits pooled, your friends will have their credits pooled, but never shall the two mix (UNLESS you are staying in the same room).
Although you will each be using your own family's credits when paying for your dining, I suggest you book one
dining reservation for the entire group. You can still divide the check between families so you each use your own credits, but if you book 2 separate reservations, they may have to seat you at different tables for your meal.
Read through all the details of the Disney Dining Plan so you are familiar with the procedure, locations, and entitlements. The more you know ahead of time, the more value you can get for each of your credits.
Wishing all of you a magical vacation to Walt Disney World!
Cathy