Salat al-Jamaat at means praying in congregation, with one persone leading. The Person who leads is called the Imam, while those who follow him are called the Ma’mum or Ma’mumin (plural). As for the daily prayers, the minimum amount of people needed is two, one Imam and one Ma’mum.
For the Salat al-Jumu’ah (Friday) there must be a minimum of five people, including the Imam.
There is no Salat al-Jama’at for the Mustahab Salat with the exception of Salat al-Istisqa ” or the invocation for rain; and for the Salat of ‘Bid, which are offered on the day of ‘Bid al-Fitr and ‘Bid al-Qurban, which are mustahab during the time of occultation of the 12th Imam (may our souls be sacrificed for him).