Two terms come to mind.
I'm deliberately omitting Get Together as it's been mentioned in other answers.
Dinner parties were a thing in the U.K. back in the day (1950's onward). They were seen as a means for social climbing, including impressing a boss (maybe an Americanism?).
They tended to have an element of formality (because you can invite people outside of your immediate or extended family it was necessary to have some kind of "extra" formality than normal).
These days, everyone will still understand the term but I don't think it's really associated with social climbing any more. Dinner parties still happen but they are certainly more informal and the "purpose" of the parties has definitely changed - they are much more to do with friends rather than family. A trite example would be hosting a dinner party to introduce two friends as a blind-date.
If you were to host a dinner party in the U.K. for friends and family I don't think anyone would be confused by the idea but it might be seen as an anachronism (everyone would definitely expect you to have big news, like a wedding or a baby for instance).
A Soirée is in layman's terms like a dinner party but with less food, and more booze! They can also just "happen" as they require less of a plan.
We all met up with Julia and ended up going back to her place for a soirée!