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.