I think the whole clique/friendship group comes down to the personalities involved. I'm quite prepared to accept that there are groups who get a bit of a kick out of being the "in crowd" but I've personally not come across this at the school gates.
I'd read so much about the cliquey school gate mums on MN that I was quite paranoid about DS1 starting school, but I've actually made a small group of genuine friends there. I'm not very good at making friends in artificial situations - e.g. toddler groups etc. I tend to start talking to someone spontaneously somewhere random, like soft play or in the park, and hit it off, rather than deliberately setting out to meet new friends, if that makes sense.
I met one other school mum outside school and found we shared an interest. She introduced me to another friend of hers, through whom we met another one. I then started chatting to another mum who turned out to share the same interest, and to be friends with one of the others. We all got on, went out for a drink and are now friends.
We always gravitate towards one another in the playground as we all do different school runs so don't see much of one another, and therefore make the most of the opportunity to chat and catch up. I wonder if some people think we're a clique because we always stand together and talk to each other.
I think there probably are genuine cliques out there, but I do think, reading some posts on MN, that some of the groups being described might well just be a handful of people who are friends, and who want to talk to each other when they see each other.
If I was aware of someone who was new and didn't know anyone, I would probably say hello and make an effort, but I have slight face blindness, so I'd probably be entirely oblivious. There are 60 people in DS1's year, and I know about 8 mums by name, another 4 or 5 by who their child is, and vaguely recognise maybe half a dozen more. I could probably pass the rest in the street and not recognise them, or have a faint sense that I'd seen them somewhere but not know where.