Take in consideration that a group walks 30% slower then an individual.
I am looking at your plan , from Astorga to Santiago it's about 270 km =average of 24 km a day which is quite a distance for a group. I don't know what time you land in Madrid and what time you take the bus, can you add more info?.
Rememebr to book seats on the ALSA bus from MAdrid.
I suggest you start from Ponferada, it's too tight schedule and long daily distance if you start from from Astorga. Starting from Ponferada will enable the group to have at least one "zero" days which is a must.
This wil also enable you to have a day rest ant tour of Santiago.
The same bus that arives to Astorga continues to Ponferada.
if you do wish to pass by the Cross de Fero you get of the bus in Astorga and take a taxi from there to Foncebadon, 5 km before the cross. There are many taxi services in Astorga nadd you can call them before leaving, the price 2009 was 34 Euros per taxi.
more questions: what time is your flights from madrid back home? how are you planning to return to Madrid from Santiago? book seats on easyjet from santiago airport.
have you booked at private albergues? I recommend you do so, it's better for a group , this will enable the group to hike slower knowing they have a sure bed at the albergues .