Exactly.
This is the thing though, the birthday girl was a newish friend to me. The other girls were a very definite group of six, I was different to them, not so wealthy or pretty and had divorced parents. I was in school with them but not 'in their gang'.
Her mum is actually a lovely woman and I can imagine if my friend said "well there's a girl in school who has moved recently, I really like her and we get the bus home together. I want to invite her but the others don't like her and I'm afraid they will be bitchy to her" that her mum might have said well invite her to the day part so she's not left out. She lives closeby anyway so will be able to walk home.
Myself and birthday girl became very very close, living together and travelling a lot over the years. But I was never accepted into this gang no matter how many years then decades passed by. They had a certain vibe about them; often mistaken as a girl band in the 90s, three of them went onto work in the media with one being a fairly household face now on TV.
The feelings were always the same with every subtle rejection. She would throw a celebration for anything and want her good friends there, I'd arrive looking forward to a catch up with people I hadn't seen for a few years and find myself pushed out for photos or not included in the bottle of champagne or whatever. Subtle but consistent.
I heavily disliked them but kept my feelings to myself as my friend was stuck in the middle. Then I noticed something; two of the other women had a long-term best friend who the rest of them didn't like, including my friend. There was never any reason just "I can't put my finger on it, she's just not one of us, the vibe is always different when she's there".
I told my friend eventually to just meet us separately.