As someone who has booked group flights before (Aer Lingus only), here are a few points based on my experience :
(1) There is a dedicated group booking number to call (see the website)
(2) Check the online price before you book - you should at most pay this or possibly do better
(3) Names do not have to be provided until about 2 weeks before you fly
(4) It's a bit of a lottery on booking to get the best price, if you need to travel on a particular day. Sometimes the prices go up nearer to the date, sometimes down. However, you have to book far enough ahead to ensure you get the seats.
(5) The usual non-refund rules apply.