As a very long time Magic Player, I have also seen and dealt with the MTG cliques that exist at LGSs.
I wrote an article a while back about my interaction with a player from one of those cliques earlier this year, here on deckstats forum.
This is not new. It is also not just an MTG thing. This happens everywhere in life. That is one of the reasons why changing schools for children and young teens is usually so hard. Trying to figure out where you fit in, if at all, is a trying time in life.
How about being the new person at work?
This is a life lesson everyone needs to figure out, and it rarely can be avoided. Even if you avoid it in some aspects of life, it will be there in others.
My best advice, is to hang out, and not dive right in. Watch and learn the formats they play, their style of play, see what kinds of cards they are using. Are their decks familiar from what you have seen online? Are their EDH decks crazy cut throat, or long game fun style? How do they act towards each other? How do they react to you hanging around and watching?
I, personally, would not recommend just jumping in to a multiplayer EDH match with a bunch of folks whom you don't know. The initial post for this thread shows a good example why it may not work out so well.
I would also suggest getting to know one or two of the folks who work at the store. Specifically, the ones who run the magic events, or play in them. They become a person that you now have in common with everyone else who plays at the store.
They will look at you differently if they see you having friendly magic discussions with the store folks.
If they seem inclusive, then they will ask you if you want to join in. If they do not, then keep your eye out for other folks to play with. Show up on FNM or Game Day, see who else visits the store. There is a very good chance you will find others who just want to play, or have an LGS they call home. Bring your own friends. Find out what day WOTC advertises for teaching rookies, and show up ready to show some new folks how to play.
I started being acknowledged once I beat one of their better players at a store championship, that and when they saw that my DCI number is smaller than 700000.... hehe.
I hope this helps.