I've never had opportunity to meet them because I was never invited anywhere, but my bf likes them a lot. From what I understand, they don't want anybody there except the people they know. There'll be around 30 people there plus married couples. I honestly don't get it.
Ummmm, I think he was bullsh!tting you. I think you kinda know it too, but he's being believable. You may not be the mistress, but the Other one certainly may be (or an ex-GF). He probably likes one of the gals in that group of friends... who's like a FWB... and he doesn't want to ruin his chances of not getting some NYE "play".
It's completely ridiculous that they wouldn't want a GF of his of nearly 2 years, who they never met, coming over. Totally preposterous. The 2nd worst thing besides fooling around with the other gal is insulting your intelligence that they would be that way for no apparent reason. Seriously? If they were That Much askew (which, don't bet $1 that they were), why in the world would he want to spend NYE with them, ditching his LTR Girlfriend that they reject for no reason at all? Doesn't make sense. The only way that Could have some truth to it -- is if he Bashed you among them as the "crazy GF that, I know, I know, need to break up with". That only makes it worse.
The writing's on the wall that he's been porking, or at least recently chasing a gal who's part of that group he spent time with. That's pretty much your answer right there, and the ball's in his court to prove otherwise Immediately, otherwise it's bye-bye.