Any advice will inevitably be subjective and, in any event, 19 year olds will do what 19 year olds will do.
Applying basic commonsense to safety is a pre-requisite and that includes being aware of your surroundings and not putting yourself (or your valuables) at risk.
The specifics of sim cards, transport and accommodation will happen quite naturally.
I always get sim cards for my daughter just to avoid her paying over the top at the airport. The airport has a whole host of sim card desks so it is a good backstop.
Suvarnabhumi airport taxi system is OK and I think you pay a surcharge of 50 Baht if booking at the taxi desk. Plan B, if the visitors are confident enough they can exit through gate 8 at departures, cross the road and pickup a taxi that has dropped off departing passengers. Personally I only take those who use the meter - then I tip well. The drivers are often looking for Pattaya fares and one quoted me 300 Baht to go to Lat Krabang....I know it is c. 65 Baht on the meter. When he came down to 200 Baht I told him he had missed his chance; I took the next taxi (who did use the meter) and it was 68 Baht. I paid 150 Baht.........just an example.
After that, use BOLT the cars are better and the drivers more professional.
Other specific advice would depend on their itinerary.