You don't need friends like those and definately sounds a bit cliquey.
I was until recently part of a quad of friends - we are all women who met starting the journey of becoming clean a couple of years ago. They started having odd girly nights without me and weekends away, then tell me they thought I wouldn't want to go or I wouldnt get a babysitter. It was a very fresh friendship group, so it wasn't already established. Instead of blaming and looking at myself, I thought I'd just observe our interactions (including those on social media too) for a while as ironically on the sobriety courses we were all on, we were covering some psychology, including reading cues from others.
I noticed that, like in your case, there was a main girl, the other two seemed to pander down to where as I wouldnt. The other two girls who didnt really appear close themselves seemed to be tripping up over eachother for that one main girls acknowledgement, time and affection. I noticed I was getting left out of arrangements when one of the panderers would gravitate towards me. It was almost as if my presence was annoying to main girl and she thought she would lose her two a lickers who were always at her beck and call, they would literally drop any other commitments they had of their own, if main girl had one of her "bad days" and needed her child taken to school for her for example, she once also threw a paddy because I refused to pack up an overnight bag for me and my child because shes fallen out with her latest boyfriend and cant be on her own in the middle of the night, I was only asked because one of the other panderers who lives the next street from her was on holiday and the other she couldnt get hold of. I eventually stopped wasting my time and noticed theyve made no effort to contact me. I left it as their toxic triangle! I bet if you took a step back this friendship group might not even be as close as appears! Wether it was established before you joined or not.