I didn't want them to think that I was spoiling their party, so I ended up going to the afterparty even though I have an early start the next day.
ご質問ありがとうございます。
"I didn't want them to think that I was spoiling their party,"＝「彼らにノリが悪いと思われたくなかった」
"so I ended up going to the afterparty"＝「だから、二次会に行ってしまった」
"even though I have an early start the next day."＝「明日は朝が早いのにも関わらず。」
ご参考に。
アメリカ合衆国 
