What is the ruling on performing Tarawih prayers communally for ten days beyond the month of Ramadan, and is the one who does so considered an innovator (mubtadiʿ), and what is the ruling on praying behind him?

It is permissible to perform voluntary night prayers (Qiyam al-Layl) occasionally in congregation outside of Ramadan, as the Prophet (peace be upon him) did so. However, persistently doing so and making it a habit is disliked (makrooh) by the Hanafis, Malikis, and a group of Hanbalis. Congregational prayer for voluntary (nafl) prayers is disliked by the Hanafis and Malikis because the nature of voluntary prayer is to perform it individually. As for the Shafi'is, congregational prayer for voluntary prayers is not recommended by them, but it is not disliked.

Designating specific days for worship in a manner not prescribed by the Sharia is an innovation (bid'ah). Therefore, the Imam who does this should be advised. If he does not respond and his innovation is not of a disbelief-inducing nature, it is not permissible to abandon praying behind him if that would lead to neglecting Friday prayers and congregational prayers.