To follow-up on newfydog - if you are renting bikes then they should be checked BEFORE you leave Leon. If you have your own and they have been packed for transport then again they will need check. There are two cycle shops that I can recommend (personal experience) : Bicicletas BiancoTeniente C/Andres Gonzalez 1 Leon (telf 987209610); and in Astorga at Plaza del Obispo Alcolea 14 (telf 987 615 882). Ponferrada and Villafrance del Bierzo also have good cycle shops. When you get to Santiago Velocipedo (Rua San Perdo 23 - telf 981 580260 - on the Camino route into town, again they were most helpful (advised me were to get the bike jet-washed and also packed it up for trans-ship back to Madrid).
More good advice about having someone with first aid knowledge - falls from bikes are the norm (although I managed to avoid them). Although most large towns have a medical clinic or pharmacy there are places that are very small and lack such facilities so you will need a first aid kit (including one that has splints for bone fractures). About the group size - if you all come from the same area then a " get-together" to discuss some of newfydog's suggestion is recommended - otherwise arrive a day early in Leon and get these issues sorted. Enjoy the ride - there is some really great scenery (as long as the clouds cooperate). Cheers