Monbus has a once daily bus leaving from Santiago to Sarria at 11AM and will pass by the airport. So you’ll have to stay overnight and get the bus the next day. The airport is in Lavacolla, it’s on the Camino route, and the 11AM bus also stops there. there are good hotels/pensions for very affordable price. Lots of places in Santiago too, off season they’re much cheaper.
Also, as it’s the weekend, the buses do sell out quickly! As used by locals/students etc. Do check now… otherwise it’ll be an expensive taxi, or try blabla car.
Buy online bus tickets and check schedules. If you prefer, hire a vehicle for your group travel. Travel in a secure, comfortable and cheap way with Monbus.
www.monbus.es
Edit: just had a quick look, Sunday 11AM bus still available, stops at the airport 11.09. From Santiago there is a bus at 1830 to Lugo that then can transfer to Sarria but you’ll have to take it from Santiago (doesn’t go to the airport). Might be rushing it if your flight arrives at 5PM.