Is it permissible for a group of men to gather regularly (almost every night) to perform the night prayer (Qiyam al-Layl) outside of Ramadan?

Performing the night prayer (Qiyam al-Layl) in congregation in mosques outside of Ramadan is an innovation (bid'ah). However, there is no harm in performing it occasionally in congregation at home, as the Prophet (peace be upon him) did. If a Muslim joins another who is praying without a prior agreement, there is no objection. As for a prior agreement to perform the night prayer in congregation on a permanent basis, that is an innovation.