Feeling ever so slightly miffed

You could show your own social graces by just going with it and making them welcome

Unless its going to cause huge issues cos you have an extra guest, i would just go along with it, unless of course you are organising a select candlelit supper

why be so miffed about it ?

You might like to wait and see for yourself if this person has social graces (dear lord, who says that?). They might be an absolute delight. Heavens, it's a party - what's one more guest? Hope it goes well.

Is this a formal sit down dinner party or informal buffet type? What relationship does the +1 have? Who told you she was immature and unsociable? Is she likely to be in your lives at other family functions?

As you say it’s a getting to know you function I think you would have to decide whether it would greatly offend and if so let it go. No point in causing ruffles simply because you are miffed they didn’t ask first.

Just say you don't have room at the table for more people and you'd love to meet this other person on another occasion

If it’s a getting to know you event, you’re getting to know someone else and may learn more about family dynamics. You might like them too, so win-win in my view.

What does your OH say about it?
Is it normal in his family to do that?

It is up to him to fix this if you two think something should be done

My sister brought yet another "serious other" to a family Christmas party one year. He was interesting, in that he helped himself to a present under the Christmas tree that was obviously a bottle of alcohol; drank until he fell down and then threw up on the front path on his way out! Funnily enough, we never saw him again.

Esther, your post reminds me of my brother's Golden Wedding party. His grandson arrived with some rather self-important American teenagers they were hosting who were actors performing in primary schools around the country.

Oh Boy! Were we entertained.

My very hospitable Sis-IL wore a fixed smile for the whole afternoon. The rest of us just wished it would soon be over.

They should definitely have asked you, just common courtesy, and you’d have said yes of course. Just be welcoming now and you’ll put yourself in a wonderful light. We were once asked if it was okay to bring some summer students to a family party as they were lodgers (paying lodgers) for the school holidays. This meant 6 surly teenagers whom we’d never met, nor would likely to meet again, sprawling around the lounge during the party (apparently they could not be left unsupervised- so if they couldn’t come to our party neither could our relatives who were ‘hosting’ them for the summer.

IMOIt is impolite to bring another person without checking with you first.
You could contact the guest to ask who the third party is. Get a conversation going, if you aren’t happy about it ,just say you are unsure about the Third Party & that you will need to think about it& get back to her.
It should then be easy to ring back & say on this occasion the invite is just for her ( your friend)

Sounds like the extra person might be a lot of fun and break the ice. Could be a bit deadly otherwise - far too contrived.

I'd say nothing. Just go with the flow. You may find you like her when she arrives. If your OH isn't bothered, I certainly wouldn't worry about it.

It sounds like this is your first challenge in melding two families! I'd be gracious about it and say nothing.

Maybe you have come across as a welcoming person so they didnt feel the need to ask. What would you have said if you had been asked?

Put yourself in a good light and be welcoming to everybody.

"...has no social graces"? How very snobbish

Just let it wash over you IMHO

Is this other person a relative, maybe ac of one of your guests, if this is the case then I see no reason why she can't come. Just because somebody is immature is no reason to invite her.As for social graces, sorry but this sounds like snobbery to me.

Why not ask WHY she needs to bring her along. I was once invited to a family social event but was asked not to bring my daughter(she has learning difficulties) as it wasn't suitable for her so I didn't go

I agree let it go! You may find you actually like the person involved.

has no social graces

Ban her now! Mrs Bucket

Oh I agree with kitty let it go not worth bothering over
I m presuming by saying one guest is bringing another you mean their partner girl/boyfriend of the moment so what’s the harm ? Yes they should have asked of course, bad manners not to but not worth making a fuss over and pretty churlish to uninvited them in my opinion Have an open mind they might be nice

Just a tiny bit snobby perhaps?