回答
This is not a piece of advice to you I'm talking to myself for my practice of writing skills.
I'm sorry I'm not good at English Grammar so I can tell whether the sentence is correct or not.
I suppose the word 'already' means something had happened before I knew him. then the sentence should be written thus.
I didn’t know him before the party, but he had already added me to his friends list.
the tense of the sentence ( 1: I didn’t know him before the party,) is past tense. next event happened before ( 1:). the tense of the sentence ( 2: he had already added me to his friends list. ) should be written in past perfect tense.
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I didn’t know him before the party, but he already added me to his friends list.
文法は正しいですか。間違っていたら何がどう違うか教えてください。