I arranged for 3 friends to meet us in Sarria in 2017. They flew from the USA.
After much research, it was easiest and fastest, to fly to Madrid with a connecting flight to SCQ. They then split the cost of a hired car to Sarria, which at the time cost about 140 euros for 3 people. If you got larger cars that took four or more it might be more economical. In the end it was much more convenient for them to fly to SCQ, via MAD and take car service, to Sarria which you could easily arrange in advance. The difference in price would likely be minimal.
I do not know whether anyone plans to send their belongings ahead each day, but taking a van/shuttle of some type to your lodgings eliminates the problem of having to initially carrying extra baggage from the train or bus station after a verylong night/ day. Some elders may have jetlag and saving them that extra push initially maybe worth it!
Our friends were at SCQ by 11:30am and in Sarria by around 13:00. They did not have to shuffle everything after a long flight. They did need to go through passport control, and customs in Madrid. There is also bus service between SCQ-Lugo-Sarria. But connections are limited.
A lot depends upon your time issues as well as your group needs. If you are dealing with an older crowd, and wanting to get there easily and quickly as possible this might be your best option.
If you take the train, you will first have to transfer to Charmartin Station, either by metro or taxi. When we take the train, we get a taxi, abt 30 + euros. You can also use the metro.
I think the quickest train takes 5hours most take as much as 6 or more hours. That also, of course depends on your flight.