I think you might be blowing it out of proportion. I mean, I wasn't there, so I don't know for sure, but I do the same thing with social stuff, especially invitations from coworkers. That's why they simply don't invite me anymore.

It sounds like you're not really close with that clique, so it makes sense that they wouldn't be discussing the outing with you, but the email was sent out and you were invited. It sounds like the person with whom you are closer friends with did want you to go, and asked, just before they were all going, whether or not you were going. It didn't sound like a fake last-minute invite to me at all. I think in a work situation where there are a lot of people involved it can be easy to fall through the cracks, especially if you're not really part of the group, and for someone who is really sensitive to this, it is especially hard not to take it personally. I get it; I've been in the exact same position and have taken it personally and I've lost the opportunity to make friends with people because of it.

I think the trap that some of us with BPD fall into is that we almost want people to "beg" us to hang out with them. I mean, it's not that we want that, but it's almost like people have to state several times that they want us around or we don't believe it and we label invites as fake or brush them off or whatever. We start looking for hidden motives. That's the feedback I've gotten in the past, at least.