It is not necessary that if you are going to the same destination and have same dates, the fares for that flight will be same for everyone. As some particular number of seats can be on the sale offering promotional or cheap fares, and once that number of seats is filled, the other remaining seats are sold at regular fares. Such as, Fare for class X(20 seats) in economy cabin can be cheaper than class Y(10 seats) in economy cabin etc. Hence, the best is served to the early birds. Also, in case where each group member has a different date to depart or return, the fares can be different too based on the availability of seats. Therefore, it is highly recommended to reserve group reservations as early as possible and make full payments well in time.