Joanna Shupe's Fifth Avenue Rebels series kicks off in a house party that takes up most of book 1 and a lot of book 2. Book 3 takes place outside of the house party, but you throw back to it in book 4. This series is incredible, and I'd read it straight through (I like book 1; a lot of people think it's the weakest, so I'll caution you there, but it really is a solid romance to me and sets up the rest of the series wonderfully).
What I Did for a Duke by Julie Anne Long (her best book imo) takes place in a house party AAAND fun games are involved!
The Wrong Marquess by Vivienne Lorret (one of my favorites of hers, I guess I just really like house parties involves a house party later in the book, I believe). The hero is already longing for the heroine after his initial dislike, it gets quiiiite heated.
Of course, Secrets of a Summer Night by Lisa Kleypas takes place in large part at a house party, where Simon Hunt and Annabelle get it going and Westcliff sees Lillian in her underwear which permanently alters his brain chemistry and sets up for their book. This is the first Wallflowers novel, and if you haven't read that series, it's an obvious must that I'd highly recommend reading in order. There's more house party action in It Happened One Autumn; not sure if it's an official house party or Westcliff just constantly letting everyone he knows stay at his house. Again the Magic, which is a prequel to the Wallflowers (and I'd PERSONALLY read it first, though it stands on its own more) may also have a house... but it's Westcliff again, so like. He just really likes letting people stay at his house.
I THINK! The Many Sins of Lord Cameron by Jennifer Ashley features a house party. In this one, the hero and heroine had a near miss years ago, and she's a lady in waiting to Queen Victoria and needs the hero's help retrieving these incriminating letters stolen from the queen. Our hero, a Massive Rake, It Should Be Said, (and single dad of a teenager trying his best) is like "SUUUUUURE.... FOR A PRICE...."
I think! There's a house party involved in Elizabeth Hoyt's Duke of Midnight, a Peak Hoyt. The hero is a duke and is courting this upper crust lady, but he's also basically Batman by night (complete with revenge and meh grip on his sanity) and the heroine finds out and blackmails him into helping her free her imprisoned twin brother. Secret affair ensues.
There is some house party action in Sherry Thomas's The Luckiest Lady in London, in which the most charming guy ever enters into a marriage of convenience with our heroine, only to be FLUSTERED when feelings ensue.