I don't know why the OP is getting such a hard time here, she said in her OP that the dynamic has changed since this woman joined the friendship group and people were PMing each other rather than going through group chat. She was a bit cheeky to insert herself into the conversation with you and your other friend @Ano1n but still being with this other friend only 20 minutes before you were due to meet with this other woman and the others was a bit daft. You should have had a different meeting place, or a bigger gap between meet-ups. (At least 1 hour.)
I would have been a little bit annoyed if I were you, but wouldn't have said anything. I would make sure I do things differently next time. (Bigger gap between meets/different venue maybe 10-15 minutes walk away,) but it sounds like you do dislike her @Ano1n and aren't really sorry for upsetting her. You clearly don't get on with her and others do, so maybe distance yourself from the group for a few weeks, and meet up with some of them but not all.
Honestly though, this is why I CBA with big friendship groups. They all seem to want to be like Friends (the TV show) and all about empowering women, and are always championing all the socialising together. but they rarely end well. A big group breaks up into smaller factions and people turn against one another, and it turns to shit, most of the time. Sometimes two of them will fall out and people take sides and it gets ugly, or the group just fractures.
I haven't been part of a friendship group for about 10-11 years now, (my late 40s,) and I never will again. Too cliquey, always a Queen Bee, always a gobshite who likes the sound of her own voice, always a bitch, always a scrounger, always a cheeky fucker, and as I say, the groups nearly all turn sour.
If the chief of the group has had a go I'm not sure how you can come back from this @Ano1n without apologising and saying something like a pp said.... Like..
@GreenFootstool · Today 09:02
"Sorry for hurting your feelings Jane, but I was having a sensitive and private conversation with someone else at the time when you wanted to join us and I tried to be as polite as possible to protect everyone's feelings in the moment.
You are not a bad person OP, but I think you could have handled this a bit better. This woman's reaction was a bit OTT drama queen though, and she has clearly been tittle tattling about you to the others. Good luck!