Hello my lovely Marmite Vipers! 
So sorry to read more stories of colleagues and school Mums being horrible. It's worse when you think you're all mutual friends, then find out they're meeting in secret huh? I agree with the pp who said it says more about them then us.
SadSongsAndWaltzes
Can I join in? I feel the same. I'm most definitely an extrovert, in that I crave being around other people and can't stand being on my own (even I think I'm annoying!) but I'm also quite socially anxious. I seem to have no filter like most people have, and will often just blurt out whatever comes into my head. I talk way too much, all about me, and then feel awful after the event and worry how I came across.
Yessss! I could have written this verbatim. So good to know I'm not alone, and this thread has proven that.
I17neednumbers
"tonight discovered they've all organised a night out together and not invited me."
Sympathies, that is a truly horrible feeling - but is it possible it's not deliberate?
Sadly not - the lady in question was showing me her phone, which had a Facebook Messenger group chat going titled "Mums Night Out" and was showing me who was on it
It was such a mixed group, hence my disappointment. I know that there are groups of Mums who live on the same estate, or have kids in the same class, etc, but the list was eclectic. Was chatting to one of them in the playground at pick up and she didn't invite me or mention it. Dh says I should casually say something in passing, but I don't think I could pull off being casual...