Are we allowed to pray Salat ul-Witr in Jamaat other than in the month of Ramadan?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Basically the supererogatory prayers, Witr prayer, etc. should be performed individually, although performing them in congregation is permissible since both practices are proved from the Prophet Muhammad (Sallallahu Alaihi wa Sallam). But usually “the Prophet (Sallallahu Alaihi wa Sallam) used to perform the voluntary prayers individually.  The evidence for performing voluntary prayers in congregation is that the Prophet once led Anas (Radiya Allahu Anhu), his mother, and an orphan in a voluntary prayer in their house.” [Reported by Imam Al Bukhari].  Al Hafiz said explaining the Hadith that 'it proves the permissibility of performing these prayers in congregation.'  Another sound Hadith proves that "the Prophet (Sallallahu Alaihi wa Sallam) led Ibn Abbas in his night prayer."

Therefore, if you offer Witr prayer in congregation at times it is permissible but doing so regularly is not advisable; it is even considered Bid'ah by some scholars.

Whenever you perform Witr prayer individually or in congregation you should raise your voice since reciting the Qur'an aloud in night prayers is likeable as long as it does not disturb other persons who are in prayer.

Allah knows best.