One of the worst things about being the "new" friend in a group is that no matter how well you click with the rest, you can never be part of an exclusive club called nostalgia, a secret land where you communicate solely with words like "good times" and "remember the days".
While I immensely enjoy the occasional trip down memory lane in the company of old chums, I know that unless someone is present the time that girl was totally checking you out from afar until you did something that was only funny if you were there along with people that have long since gone their separate ways, it gets quite frustrating for the others.
So if you're in the presence of a group of people who have known each other for close to a decade and 90% of the conversation is of fond memories, I think it's quite fair to write the night off as crappy.