ご質問ありがとうございます。
「[二次会](https://eikaiwa.dmm.com/uknow/questions/1712/)の幹事」は英語で「person in charge of making arrangements for the after party」と言えます。
また、二次会の幹事について、説明したいなら、そうすると「Two of us are in charge of making arrangements for our friend's wedding party. We will be surprising our friend with compilation of old pictures and video to show it to everyone.
In Japan, it's a tradition to make preparations for the person who will be getting married. A wedding ceremony is followed by a wedding party.」
という風に言えます。
ご参考になれば幸いです。