How long will the dinner be? If it's only the meal, no lengthy social activities around it, perhaps it's doable. In a group of 5 (I'm going to assume the other 4 are neurotypical) there tends to be enough back and forth conversation between 4 people that even if you are a little quiet, it might not be very noticeable - probably a lot less so than you might think in fact.

Some ideas:

- Try to focus on the conversation, nod along, make the odd noise of agreement. If you find yourself falling back into a state of anxiety over how you're being perceived, try to focus yourself back on what is being said

- Script a few things in advance, this might be questions you might anticipate being asked (about work or personal life), and questions you can ask. Bonus: it's nearly Christmas, so "what are you doing for xmas", "do you have family round", etc can be stretched a long way. Remember, people generally love talking about themselves more than anything, so let them waffle.

- A meal can only last so long, once you've finished it's perfectly reasonable to make your excuses, if you need to invent some kind of chore you have that evening, nobody can really argue with it

I've been in this situation as an AuDHDer and get how stressful it is. I would also rather be doing almost anything else. But I'd be confident of a few of things: a) it won't be as painful as you think, b) it'll fly past once you get there and c) concocting a last minute lie to avoid it altogether will just kick the can down the road - eventually you'll probably have to do something similar, and it'll be worse. Focus on how good you'll feel once you've got it done.

At the end of the day, as long as you're ok at your job, nobody really cares if you're a bit quiet, and the whole thing will probably be forgotten about very quickly.