"如果那天参加活动的朋友都是单身的话" => most participants should be singles, I don't think we are in the position to define their party.
"将这一活动发通知给我们俱乐部的朋友" => Now I'm embarrassed, hi hi!! I've just complained about 晴天的广告, I should not have asked u to eMail your group. :p I appreciate your offer a lot, 并相信 假面舞会的同学们 也很感谢你的心意。