I have a flight that's a few hours later than someone else in my party. Am I able to check-in at Fiumicino with them several hours early so we can spend time in the airport together? I have done some internet searching both on my airline's website and on Fiumicino's website and haven't found an obvious answer. I'll only have a carry-on. Thank you!
I fully understand your question and have faced checkin starting times and also baggage drop-off times in the past. It is possible that the answer may be more airline specific than airport designation.
If you need to drop off bags practically all airlines will let you do it 3 hours prior to departure for Extra-Schengen flights, but not sooner. If your flight is Intra-Schengen, then it's 2 hours. It depends on the airline however, some may let you drop off a bag up to 4 hours prior for an extra-Schengen flight. Never had experience with very early drop off at FCO, but in some airports I tried, but absolutely no chance. They won't let me do it before 4 hours.
If you don't need to drop off checked bags and you checked in online already and have a boarding pass in your phone, you may be able to go through security pretty much when you want, even beyond 3 hours sooner.