Believe it or not, the problem may be you, not them
People complain about perceived cliques all the time. From mom's groups to neighbors in apartment buildings, work teams, or kids' swim teams, a frequent refrain is, "That group is so clique-y" or, "I felt so left out."
Further exploration, however, reveals a basic misunderstanding of the social structure of a situation.
If you are a new person seeking to join an already-established community, whether it be a neighborhood, school, workplace, volunteer group, yoga class, book club, or sports team, it is only natural to feel left out at first. If you enter a situation where others already know each other, remember just that: The others already know each other. This can make it difficult, but it does not mean the members are bad people. They will likely seek out and talk to each other, have inside jokes, and actually be interested in each other and sit next to each other. They have pre-existing relationships, but that does not necessarily mean that they are a clique or that they do not want to meet you.
In fact, it's helpful to know that this is a community that actually builds relationships. Which group would you rather belong to—one in which the members don't really associate with each other, or one in which they have built a sense of cohesion through supportive interactions?
People who already know each other, and who join in conversation together, do not a clique make. There is no doubt, however, that adult "mean girls" exist. In my studies of friendships, I have encountered many stories of deliberate exclusivity, snobbishness, or even social aggression. Yes, certain groups are cliques, just like certain members of many groups are superficial, judgmental, or uninteresting. Often, however, it's nervousness from both parties that make a group's seemingly icy boundaries hard to thaw.
In fact, as has been the case with many people with whom I've worked, the pre-existing idea that some group is a clique is often a self-fulfilling prophecy. If you think you won't be accepted into a group, you are less likely to try to strike up a conversation with a person you haven't met, and then you yourself may become the "snob" with the pursed lips standing off in the corner. You will be more unapproachable and reinforce your own belief that everyone else thinks they're too good for you.
I must admit to a certain fascination with the accusation of clique-ness in groups that one has not even attempted to join yet. Let's say there is a gathering of an organization of which you are a brand-new member. What are the already-existing members who already know each other supposed to do, take a vow of silence and not acknowledge or talk to each other while they stare at the entrance to see if someone new is coming, and then immediately bombard that new person with praise and attention?
Veterans of any organization or community should, of course, be on the lookout for newcomers. They should go out of their way to try to make conversation with someone who is alone or looks ill at ease. Those of us who are new a group also have a responsibility, however, to challenge ourselves to approach others and give them a chance. Make eye contact, ask open-ended questions, display welcoming body language, and acknowledge your interest in the conversation through pleasant feedback and nodding. Shyness is not the same as snobbishness, just as established groups of friends aren't necessarily cliques.
If we were all willing to step out of our comfort zones and talk more to people (and put down the safety shield of our smart phone while we're at it) we might discover that it's not really about cliques, but about clicking.