I remember a few times in grad school, when I was the new student, being invited to join a small group of people for lunch. They would talk about the fun past weekend and their fun plans for the next weekend or whatever, without offering me an invitation for the future or acknowledging that I hadn't been invited to the past things.
I figured if they invited me for lunch, maybe they would want to hang out at other times, but a few attempts to say things like "Oh that sounds fun" or "I should try that place sometime" with no effect, I quickly found other stuff to do at lunch. Who needs to hear about how you're being left out the rest of the time? I found out much later that they didn't invite me to other things because I was friends with someone they didn't like, and they were afraid I would bring her too. Nice.