パーティーでは

at the party

→ パーティー
phrase Uncommon #16,218
at the party では combines で, marking the location or setting, with は, adding emphasis or contrast. Here it means “at the party” or “as for at the party.”
JA: パーティーでは何を食べましたか?
EN: What did you eat at the party?

彼女の服装正式なパーティーでは場違いだった

Her outfit was inappropriate for the formal party.

パーティーでは誰もが楽しい時を過ごした

Everyone had a great time at the party.

パーティーではご機嫌だったな

You seemed pretty cheerful at the party.

— Hoje Eu Quero Voltar Sozinho (2014)

パーティーでは何が出された

What was served at the party?

パーティーでは子供たちは寝ずに起きていて行儀よく振る舞った

At the party, all the children stayed awake and behaved well.

そのパーティーではだれも美しい着物を着ていた

Everyone at the party was wearing beautiful kimonos.

キャンプでは飲んで記憶は残ってたカリーナパーティーでは記憶が飛んだんだろ

Even when you drank at camp, you still remembered everything. But at Karina’s party, you blacked out, didn’t you?

— Hoje Eu Quero Voltar Sozinho (2014)

パーティーではたくさんおもしろいに会いました

Did you meet many interesting people at the party?

そのパーティーでは行き渡るほどの十分なワインがなかった

There wasn’t enough wine to go around at the party.

パーティーでは泥酔だったのにのこと?

You were dead drunk at the party, so what are you talking about?

— Hoje Eu Quero Voltar Sozinho (2014)

パーティーでは大いに楽しんだ

I had a great time at the party.