To ask why people do this?

Agree wholeheartedly.

This is what I’d be saying

Why can’t you send the message @LoveSandbanks suggested now?

Your friend is rude or inconsiderate or both.

Cane here to say this. Life's too short if you don't want to go then just say it. Annoying yes but she'll either get the picture or not.

Aw OP I had this with a friend’s husband. You’d think he wouldn’t want to come and be the hanger-on but he kept popping along.

Once she said he wasn’t coming because I made it clear I wanted it to be just us two, then he showed up! He’d come to all girl lunches as well, the only partner who was there, all of us with our normal self-aware husbands at home.

I ended up being a petty bitch and just chatting away to her about in jokes and other things he wasn’t a part of and I think he got fed up of being the third wheel.

This. Life is too short to spend your spare time doing things you don't want to do. Be an adult about it and be honest. You'll know if she's a true friend from her reaction.

Sometimes this is just a sociable person being thoughtless or lacking social boundaries, but it can lead to excruciatingly awkward situations.

Many years ago I had a series of coffee catchups with a friend, who we will call Sally, at her home. She would invite me for what I thought was a 1:1 chat, then once I had arrived would say ‘oh, X is joining us and will be here shortly’. Three times in succession, she invited X, Y or Z as an extra person without telling me beforehand. Then I would have to quickly recalibrate possible chat, while X Y or Z would arrive and look very surprised to see me as they’d also thought they were there for a 1:1 catchup! Just so awkward. I slightly knew X, Y and Z but only very tenuously through local circles - all perfectly nice people, but I wouldn’t have prioritised spending time with them for a casual coffee when my free time was very limited.

On the first occasion, Sally cheerfully asked about X’s very personal medical issue in front of me. On the second occasion, it turned out that Y was someone I had once worked with. On the third occasion, Sally chatted brightly about what she thought was a lovely connection between me and Z (had both once been members of the same large hobby group), blithely unaware that Z no longer attended that group partly because of a family bereavement.

Eventually I realised that Sally saw herself as a social networker locally, and this was her way of trying to connect people she thought had something in common and creating what she thought was a jolly group chat. But what she didn’t have was the social awareness to let people know that there would be other people joining, and to avoid touching on personal or sensitive matters that should have been kept for a 1:1.

After this happened three times I said to Sally that I wasn’t comfortable being presented on the spot with people I hadn’t known were coming, particularly if it led to awkward chats on sensitive topics, and that another time, I wanted to know beforehand if she had invited anyone else.

I don’t think Sally ever really understood the nuances of this and eventually I just withdrew from meeting up with her! I think some people are just very lacking in social antennae.

When someone sets up a 1:1 with you, it usually means they want to spend time with you, just you. That you matter enough to hold that space on your own. So when they confirm the plan on the day and then suddenly mention they’ve invited someone else to join, it shifts the tone. What felt like a personal catch-up starts to feel more like a group hangout, and you weren’t really asked if that was okay.
It can leave you feeling like you’re not important enough to be seen on your own. Like they’re trying to tick off two social boxes at once. Like you didn’t quite make the cut as someone who gets time carved out just for them.
It’s not just about the change in plan. It’s about what the meeting meant to you versus what it clearly meant to them.

We all want to be enough without needing to be padded out with other people. Personally, I wouldn't go.

This

This is perfect

Going against the grain here to say that unless I had something deeply personal to discuss with the friend, it wouldn't bother me

I’m also too old for this shit op. I had a friend who did this, and so I just never met up with her one on one. I also have no interest in chit chatting with acquaintances.
I would absolutely decline now because she clearly knew you didn’t like it last time and did it again.

The first and only time someone did this to me was when I was 20. Home from university at the end of my 2nd year, my best friend from school (who hadn't gone to uni), whom I hadn't seen for several months, arranged to meet me in our city - and turned up with a girl I'd never spoken to before (knew of her as she was in the year below us at school). They mostly chatted to each other all night.

Never saw my friend again. I don't need it in writing!

100% this

This

Just do this!

no more rude than her inviting someone else along.

"Joanne's lovely but I was hoping to catch up just the two of us. I'll bow out this time."

If you can't genuinely say this to her then you need to drop the friendship. It's as simple as that.

I have a friend that does this EVERY single time. We’ll arrange a day with just us and our kids she’ll always invite someone else along, sometimes two! It totally throws off the dynamics of the friendship as they’re people she sees probably daily where as I don’t and it usually means we then have to find somewhere to accommodate 3+ adults and 10+ kids when it’s time to eat. The last time she did it I politely declined and offered to meet her another day. I’ve told her before I don’t do well in massive social settings as they’re quite open when sharing relationships etc where as I’m not comfortable with that and prefer smaller circles. I’ve not arranged to meet her in months but to be honest she’s so naive I don’t even think she realises it’s because of what she does.

Be the bigger person, suck it up this time but on the next occasion that you are planning to meet up make is perfectly clear that you want it to be just the two of you and you don’t want a third person tagging along to change the dynamics. If you don’t communicate nothing will change will it?!

'But it's not really friendly...'

Not the OP's problem

Why are you so passive? You don't need to lie or find an excuse for not going - why would a person be so disingenuous anyway? Just tell the truth! Don't complicate it.

She asked if you're still up for meeting her - you said yes - she's told you Joanne will be coming - you're not up for meeting with Joanne - so you pull out. It's literally that simple. What's flakey about saying "oh you didn't say you were bringing Joanne. I was hoping to see just you on Saturday. Not to worry: you go ahead with Joanne and we'll find another date that works".

It's really not that deep. Are you that much of a people pleaser that you'd sacrifice your time to placate Joanne and this friend to your own detriment? No wonder it keeps happening to you.

This. Unless you are struggling w something you wanted to talk to your friend about would it kill you to get to know someone new? Don’t worry guys, I know the answer! The UK is very unfriendly it’s not like this in other countries.