Unfortunately I have encountered some very malicious cliques in my time. Most recently at a school where I was a teacher's assistant.
My colleagues would talk in the room as if I were not there. They would also make that obvious to the students that I was responsible for and that caused them to not listen to me when I gave directives. I did not fit into the clique and eventually ended up leaving as after 3 years of their targeted mental abuse of me, I nearly had a mental breakdown.
If you see someone on your work team who is on the quiet side, don't leave them out. Don't talk as if they are not in the room. If the person is from a different culture, don't use that as an excuse either. You are there to WORK and not just on your social life. You are paid to do a job and you are to get along with your colleagues.