Leader of the Congregation

Abdur Rahman bin Abd Qari narrates that one night, during the month of Ramadan, he went with Hazrat Umar (ra) to the mosque and saw that people were scattered in various groups whilst offering their prayers. Some were offering the prayer on their own, whilst others were offering prayers with a small congregation behind them. Upon seeing this, Hazrat Umar (ra) stated that he deemed it better if everyone was brought together as one congregation behind a single qari [Imam]. Upon making this decision, Hazrat Umar (ra) appointed Hazrat Ubayy (ra) bin Kaab to lead the congregation.

(Sahih al-Bukhari, Kitab al-Salat al-Tarawih, Bab Fadl man Qam Ramadan, Hadith 2010, Translated version, Vol. 3, 680-681, Nazarat-e-Ishaat Rabwah)

In other words, the Muslims were offering their nawafil [voluntary] prayers at night.