When we were getting certified at ADM there were a good few people doing their referal - you dont really have to book anywhere as far as we could tell. When i went back in Dec my friend got certified, i went along on 3 of her four dives (stayed on the boat for the other one) probably best not to get too involved if there are a group of them and they are doing their skill - especially if the instructors dont know you or have never seen you dive. Highly recommend Amigos - Eddie is a great instuctor (as is Jim).