What should one do who enters the mosque for Tarawih prayer and finds the people in the second rak'ah?

It is permissible for someone performing a fard (obligatory) prayer to be led by someone performing a nafl (supererogatory) prayer. So, if you find an Imam leading the Tarawih prayer and you have not yet prayed Isha, you may join him. When the Imam says the taslim (salaam at the end of prayer), you should stand up and perform one rak'ah with Al-Fatihah and a surah. Then, you sit for the middle tashahhud, then stand up and perform two rak'ahs with Al-Fatihah only. This is because what the latecomer catches up with is considered the beginning of their prayer.