I'm sorry this is happening. Look: cliques thrive on the thrill of excluding the people who are not "in" them. It's basically seventh grade with 25 year olds. Also, people who will do this at 25 will still be doing it at 70 - the level of adult bullying in old age homes (oh, I guess we call them "retirement communities" now) is just amazing. They never grow up, they never turn into better people. Many of them, anyway.
You "win" this by not caring - you can make sympathetic noises, say "oh that's too bad, I'm sorry you have to deal with that" but not reach out, not extend yourself, and you'll take all the fun out of rejecting you because you are no longer there to reject. Even if they talk about some social thing they're all planning, or have already done, without you, and they're gauche enough to do that in front of you - you can shrug, say "hey, sounds like you guys had fun" , refill your coffee mug and go on about your business.
It will feel strange, and you'll possibly feel as though *you* are being the bad guy by not trying to help and not trying to join their social circle. But they've already told you what you need to know. People who enjoy excluding others on purpose are just not nice people. No matter how they gush over one another!
It's OK to leave them to their own tender mercies, but in a workplace it's wise to make positive-neutral noises about their fun times, often enough so they can't claim that *you* are somehow snubbing *them*.
And yeah, ugh.
_________________
"I believe you find life such a problem because you think there are the good people and the bad people," said the man. "You're wrong, of course. There are, always and only, the bad people, but some of them are on opposite sides."
-- Terry Pratchett, Guards! Guards!