Most of us have been guilty of it at some point in our lives. We don't mean to---but we unintentially exclude others from our group.
I have gone to the same church for 24 years and have friends there that are practically family. We know more about each other than anyone probably should. Visitors haven't always felt comfortable attending our sunday school class because they feel left out of our closeness. They don't know the inside jokes and so they feel excluded.
It's easier to talk to people we already know and harder to get to know new people. I think this is the real reason we forget to make others feel welcome and included.
I'm fairly out going and make myself at home just about anywhere. It takes a lot to deter me from a new group so I sometimes forget to go out of my way to make new people feel at home and welcome.
Sometimes we just need to go that extra mile, take a little more effort to make a shy, introvert person feel needed and cared about. :0)