OK, dinner party sounds a bit poncey, but I had everyone in my office round for dinner the other night. It was specifically for my team, no spouses, partners or whatever. I have a friend who works in the same building but in a different office, who wasn't invited to the dinner (she comes round quite often for drinks/dinner etc anyway) as she has nothing to do with us, workwise. Anyhoo, she bumped into some of my colleagues on the way out. They said they were coming to mine. She said, oh great, I'll come too, I've got nothing else on. She texted me and said "FYI I'm crashing your dinner." Unfortunately I didn't pick up the text until about 30 seconds before she arrived, so I couldn't say, no you're fucking not.
I was extremely pissed off about this, and was a bit frosty with her (but she is quite thick skinned, so probably wouldn't have guessed), but AIBU to think that this is really bad manners, and something you just don't do? I mean, if I had wanted her to come, I'd have asked her, FGS.