Which is better for a group that has not yet prayed 'Isha'—to join the mosque's congregation that is praying Tarawih with the intention of praying 'Isha', or to establish another congregation for prayer?

If a group enters the mosque while the Imam is praying Tarawih and they have not yet prayed Isha, it is permissible for them to follow him with the intention of Isha, and their prayer is valid. They will also receive the reward for congregational prayer, according to the more preponderant of the two opinions among scholars.