I love Phillips Old Colony House on Morrissey Blvd in Dorchester, just over the Quincy line. Easy to get to off 93, they take reservations for a group, they have an awesome great variety buffet (I'm fussy), good service and a bar. Enjoy!
Don't know if they are already booked up or if they can accommodate your group, but I have always enjoyed the brunch at Henrietta's Table in Cambridge! It isn't your typical buffet, but the food is great!