One possible version：
Hi Susan，
We're_throwing_a_surprise_party_for_Xiaoming's_birthday. We'd like to invite you to the party. We want to give him a surprise, so please keep quiet about it. We'll gather at the Students' Club at 8 pm this Friday, after the evening classes. And it's going to last about an hour. We each will prepare a little present. When he comes, we'll light the candles and sing “Happy Birthday” together for him. Then the cake will be cut and we'll sing songs and play games. It'll be nice to see how excited he will be. I'm sure you'll have a good time at the party, too.
