Friend invited her friends to my birthday meal

Your friend made it awkward by asking in the first place. The onus shouldn't be on you. I can't understand why you are getting so much flak for this.

She knows you are shy and not comfortable with people you don't know but it sounds like she's invited more than one of her mates and their husbands at the last minute. If she's also bringing her DH... you friend is proposing to bring her own group of six to your birthday party. ? (My assumption of course) I wouldn't be happy about that either. It would change the whole tone of the evening.
In effect, your friend is making your birthday all about her and her guests and you are not happy about it.

You don't say if its at your home or in a venueI guess it depends on How much will you have to change and how much will it cost you to add them. If its very informal, it may not be such a big ask... but if its a restaurant booking or per head it may be hard to squeeze in an extra four in a busy room. In that case your friend is asking you to pay for four extra meals which is cheeky.
People often quote the phrase "It's an invitation not a command"... in this case, its an asking for an invitation,but that doesn't make it a command.

Only you can decide if you want to include these people or not. But its already causing you anxiety. You could ring the friend and explain that you'd rather she and her DH still attended, but could she ask her visitors if they could find something else to do that evening? Maybe the could come afterwards for drinks instead... that might be a good compromise.

However, whether you do or not. It does sound a bit like it was sprung on you and you couldn't think of a reasonable way to say no, although your gut reaction was no.

It depends if this happens a lot with this friend.Is she the type of friend who makes it a command or you have learnt that her friendship is conditional on you going along with her suggestions?
You need to practice buying time and having a chance to think about your answer. You DO NOT HAVE TO SAY YES IMMEDIATELY if your gut is telling you no.
Make a point going forward of saying..
I will think about it and get back to you... so she doesn't put you on the spot in future... or let me check my diary and I will let you know. You never have to give an outright yes or no if you don't want to.

She sounds a bit cheeky and I'd be asking
"Does that mean you won't come to my birthday party, which you accepted an invitation to x weeks/months ago unless I agree? Because if she says yes.. that's a bit of a blackmail isn't it.

She knew in advance before the visitors arrived that she was booked for that evening and she should have told them and they could have worked out an alternative. Also depends if the visitors are only there for a night or a week. A night,then its not so bad to ask, a week - not the same.

It depends what she's like normally. You said she knows its your first birthday celebration in 10 years. edited to add.. and you've had long conversations with her aobut your anxiety and doubts about the party but decided to go aheadd anyway... So she KNOWS how you feel about it.

And she's asking for you to cater for extra people when you've already made the plans for your party. That may be easy to fix, but it may not. So those things need to be taken into consideration.
Listen to what your gut is telling you about what you can reasonably accept and what you can't and think of the best way to go forward. When you know what you ask is reasonable and what the other side is asking is not convenient or not as reasonable, it makes it easier to tell people what you want to happen.

If you’re going to an event for the kind of person who just doesn’t mind people rocking up and loves meeting new people. But the OP is clearly not that person. As she says she knows she’s not that good at saying no to things. She didn’t even want this dinner but having slightly reluctantly agreed to it she’s stuck with something that will make her feel anxious. On her own birthday. That’s not right.

Empathy is not imagining yourself in a situation where you would feel a certain way and sympathising with the other person on that basis. Empathy is putting yourself in the other person’s shoes and understanding how they would feel. And the OP’s friend is just not doing that.

Personally I think she should go back and tell the friend that she doesn’t want her to bring her friends and why. I get why the OP finds that really hard. But I also think she will feel a lot less anxious if she does.

You’ve done well to tell your friend the truth about your feelings on the situation. That must have been hard, but it was the right thing to
do.

The stress that arose from your friend asking the question is your responsibility. You shouldn’t blame her, but instead need to work on your responses. I have a phobia of dogs. Walking in the park causes me massive stress. I don’t blame the people walking their dogs for causing my stress, because I understand that it is my issue.

Hopefully being honest with your friend, and the world not ending, will give you a bit more confidence to build on your assertiveness in the future.

I didn't say it wasn't.

I was replying to the person who said her friends options were "either pulling out of your dinner or asking if the friends could come," missing out the third option which was to inform the guests that she has already has plans and will see them later.

Your friend isn't a mind reader and I think she was trying to make the most of an unfortunate situation and be considerate of you by asking you.

To be honest you've tied yourself in knots here with wilder and wilder excuses trying to justify your feelings. We all have feelings, we can't help them, that's fine. But that doesn't mean that they're always reasonable and how you deal with them is on you.

I'm not disputing the birthday means a lot to you.

I'm disputing that it's ok to have your "biggest grievance" that she did not anticipate your reaction completely. She's your friend but she's not a mind reader. It's ok that she perceives things differently

Asking if her friends could join is a potential solution.

that’s my biggest grievance

This seems a huge overreaction.

She was invited to your birthday dinner. She is having house guests arrive unexpectedly a day or twoearly so she texted you to ask if they could come along. You weren't even put on th spot by her asking in person.

You've now turned her into an uncaring person who should never have considered asking because she's expected to know in advance that she's (disproportionately) stressing you out. And that your "culture" means you have to lie rather than say you'd rather not.

That's both unhealthy and unreasonable.

But you are the one who's created that situation. You only have yourself to blame.

It's good you stood your ground and now she understands your boundaries.

I guess for some people the more bodies in the room the better. I'm much more of an intimate friendship type person. I can imagine you're similar.

But some can only seemingly socialise in large groups. And think nothing of putting very different people who are sometimes strangers all together!

I can see it's hard with the people pleasing and social anxiety. If it will really spoil it for you just send a polite heartfelt text, briefly saying you've thought about it and it will cause you anxiety so you'd rather not. Any decent friends would understand that and entertain themselves for a night if your mate just said it was an intimate do that had been booked for ages.

If I didn't want to I would've said no.

I don't see why it would necessarily make your birthday celebration worse? If you find them not to your taste you just speak mainly to your actual friends. They could be lovely and add to the fun atmosphere? Presumably they'd pay their own way?

But If there's limited space and it's meant to be more intimate then that's a perfect reason to politely decline.

I completely get you OP. You understand yourself very well. You’ve explained it to your friend. She’s appeared to understand it and then she’s done something that either demonstrates that she wasn’t really paying attention or did understand but was going to ignore it because it didn’t suit her (i.e because she’d already invited these friends and it’s easier to upset them than you).

I hope you get the birthday you actually want. I always regret it when I compromise when it’s my celebration. Happy to compromise for other people’s or not go if that’s the only other option.

Maybe she thought that as you don’t have friends of your own, she’d bring some of hers along to make it more of a celebration rather than a meal for two.

As far as I can see, the only social crime OP has committed in this scenario is to say "yes" to something she would rather have said "no" which whilst not ideal, is something I suspect all of us on this thread do from time to time for various reasons.

On the other hand, bringing your own friends along to someone else's birthday meal "to make it more of a celebration" is not a normal thing to do.