That Achashveros’ party was an aveira is proof to this issur. The Shulchan Aruch (YD 152:1) paskens its assur to party with them, based on the Gemora’s in Megila and Avoda Zora.
The Taz writes that the prohibition is m’doraysa and even applies if the Gentile making the party is non-religious. The Mishna Halachos 7:118, based on the Taz, specifically says the prohibition includes attending an office party.