To not like when people invite others along?

[NoCapes]

I have a friend who every single time we plan to do stuff with the kids (we're only really friends because of the kids) will always let me know about 10 minutes before that "oh so and so from school/slimming world/the house next door is coming too"
I'm not the most sociable butterfly and it really pisses me off
AIBU?

There are times when it's apropriate and times when it's not.

I'd be happy if a friend said 'I've met X, she's really nice but is a little lonely. Do you fancy doing something together?'

Or if you're in a larger group, I don't mind it so much. Although it depends on the make-up of the group. If it's a group of old school friends who want to relive the moment JImmy copped off with Tania behind the bike shed, and wasn't Mr Brooke a bit of a pervert in hindsight, it probably won't be much fun for the newcomer.

One-on-one though is difficult particulalry with intimate friends. It just doesn't work - you can't talk about things you want to talk about, you have to avoid in-jokes and monitor your behaviour if there's someone you don't know there.

Sorry thought I was doing a new thread - and can't find delete!!!

There is no delete.

YANBU at all, I hate this

And I think it's rude

Perhaps her friend is going through a rough patch and needs a distraction, or maybe she's new in town and doesn't know many people and your friend is just trying to be nice and include her in things to make her feel welcome. She isn't inviting people to upset you, she's inviting them to be friendly to them. If it upsets you that much then why don't you talk to her about it? Presumably you're pretty good friends, why can't you just say to her "it makes me a bit uncomfortable when you invite people I don't know to our playdates and don't give me any notice. Next time could we just keep it the two of us, I'd really appreciate it?" How do you expect her to know that it upsets you if you're don't tell her and just act like everything's fine? She's only trying to be nice and inclusive of her other friends, so just talk to her about it.

"She's only trying to be nice and inclusive of her other friends, so just talk to her about it."

Yeah, you don't know that. She could just as easily be bringing someone else along because she can't deal with one-to-one with the OP, for whatever reason. NO ONE knows that.
The only bit of your post I agree with is that the OP should talk to her about it, and tell her she doesn't like the addition of randoms to their supposed one-to-one meet ups.

I am bubbly and chatty but was pissed off last Thursday when I met my friend at local park. She had brought along a friend of hers who I know to say hello to, but we're not friends. My friend explained that she'd invited her as they hadn't caught up in a while. This person dominated the conversation and talked a lot about her (first world) problems such as she hadn't had a holiday abroad since June (wtf?) and issues with space in her 5 bedroom house (??) - I like to think this person would have have been more tactful if she knew me better (no holidays and a tiny 2 bed house) as I found her comments thoughtless. And I didn't get a proper catch-up with my friend.

I am normally keen to have more friends, but on this occasion, I was a bit put out.

Had to revive this thread this happened to me and left me fuming. I'm quite anxious and one of my kids has asd so meeting up at a local farm attraction for kids means it could be fine it could go terribly, I have to work myself up to go join my friend who says she is walking down there. Luckily I go early as I'm closer to the place and as she arrives she texts oh friend 2 and her friend you have never met and their kids have joined us too.it was fine I left after another hour but seriously pissed off. Also meeting for coffee, I'm texting I'm on my way,get back a text friend 2 is here just didn't want to tell you in case you cancelled. Gah

Huh? "didn't want to text you in case you cancelled"! So she knew and admitted this would make you uncomfortable but invited other friends anyway....
How disrespectful... Urgh

I would send a stroppy text back saying "if you suspected I would cancel because you invited friend 2 then why did you invite her rather than meet up with her another time? Have decided you aren't a real friend and now am cancelling as I don't like the way you treat me"
I do think adding extras should be discussed by both parties if only 2 people are meeting up. The friend sounds selfish and totally lacking in empathy.

I wouldn't mind this at all, I suppose unless I had a specific heart and soul bearing issue to talk to a friend about. Having said that, I'd probably text and say 'do you mind if I invite Friend2' - I just think the more the merrier and it wouldn't cross my mind to object. If it was me arranging I would optimistically assume everyone would chat together and get on.

It's not that it's the element of surprise which takes away my choice of whether i want to hang out with these other people or not and thinking u are having a catch up then suddenly it's a group activity

If I was asked or forewarned at least id have the choice

This is very rude. You need to tell her that this upsets you.

I think I will next time we meet as in I'll begin by asking who is coming and take it from there