Is establishing a congregational prayer at night restricted to not being an innovation if it is not by prior agreement, or is it permissible sometimes whether it is by prior agreement or not?

Congregational supererogatory prayers outside of Ramadan are not legislated unless they are not performed regularly and no specific night or nights are designated for them. If one of these two conditions is violated, it becomes an innovation (bid'ah); otherwise, it is not. What is prohibited is agreeing to pray every night in congregation, or designating a specific night for congregational prayer. Furthermore, even designating a specific night for individual prayer (qiyam) is not legislated.