ANSWER:

You must join the Imam in taraweeh with the intention of praying Isha, when he concludes his prayer, you get up and finish your remaining two rakahs of Isha.  

 

When I enter masjid, Isha prayer is over, they started taraweeq, my question is should I join taraweeq immediately in congregation, then pray Isha at last or else I will finish Isha then join in congregation?