As a related - but possibly seperate note. Last year there were 3 of us flying on the same booking to Berlin with Aer Lingus. We were checking in baggage and had checked in online the night before. Unfortunalty i was running late and was getting frantic calls from the others in the airport from the others that if I didnt arrive they couldnt fly as we had checked in online together and they pooled the baggage.
It was a mad dash from the airport bus to the standby desk to see some angry faces on my friends - thank God I made it!
So while the booking may be made together - check in online seprately if you are coming from different locations to the airport.