"I went to the company's Chinese New Year party yesterday."
Native speakers, do you think my answer is also acceptable? " 我昨天去公司的春节聚会了。"
我 at the beginning is fine and perhaps even more natural than Duolingo's sentence, but the 了 at the end changes the meaning slightly. There are several discussions explaining 了 usage so I won't elaborate here; suffice it to say that the current tone of your sentence is more like announcing something that has just happened, complete with exclamation points!