"It is of Sunnah to perform Salat Al-Witr both single or in Jama'a (in a group).. It's the same, Regarded that you don't make it a habit. Because making it a habit is considered Bida."*
*This is my own summarized translation to a Fatwa by Shaikh Ibn-Uthaimin.
Salat Al-Witr is a single prayer (1 Rakat) performed during the time between Al-Isha prayer and Al-Fajr prayer, And must be the last prayers to be performed before Al-Fajr prayer. and prefered to be at the last third of the night.
It is also prefered to be performed after an even number of Rakaats (As in Sunnah).. where the Even number of Rakaat is named "Shaf'" and the odd (one single) Raka is named "Witr".
example: you can pray 2 rakaats as Al-Shaf' prayer followed by 1 Rakaa as Al-Witr prayer.
Al-Shaf' & Al'Witr together are called "The night prayer" or "Salat Al-Lail" or "Al-Qiyam prayer"... performed with a minimum of 3 Rakaat.