Feeling ever so slightly miffed

Ah the vagaries of families - use this experience as part of getting to know them. Then you'll know what to expect when next you issue invitations out and can then either invite, or not.

Oh dear, an unfortunate start! Yes, they should have asked first obviously. I can imagine how galling, but perhaps to avoid things getting off on totally the wrong foot, just swallow hard and make the best of it?
I can’t understand how some people are so thick skinned and oblivious of how to behave.

I would be miffed too. Is this ‘third party’ a Significant Other or a relative who can’t be left alone? Otherwise I would ask your OH to see if this uninvited extra could be uninvited.

I'd go with the flow. Dh and I love talking about 'funny' relatives. It's the stuff of family life.

If this +1 is their SO then they should have been invited, if not, then no invitation means no attendance.
From your point of view numbers may be critical and if their is an element of “formality” then it May not be easy to add another guest.
You don’t say who the guest is or who the +1 is, if you were to it might make things clearer. Bottom line is that even if they did not ask you are within your rights to say No.

Oops “there” not “their”

Just a tiny bit snobby perhaps?

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

has no social graces

Ban her now! Mrs Bucket

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

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

Just let it wash over you IMHO

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

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

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?

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

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.

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

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)

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.

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.

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.

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