The Ruling on a Lone Worshipper acting as Imam
Question :
Is it permissible to join with a single worshipper who performed the prayer alone?
Answer:
Yes, it is permissible, based upon the authentic narration of Ibn 'Abbas, may Allah be pleased with him, who said: "I stayed with my aunt, Maimunah, may Allah be pleased with her, and the Messenger of Allah stood up to pray the night prayer, side." so I stood on his left side, but he pulled me to his right.
And essentially, in such matters, there is no difference between the supererogatory prayer and the obligatory.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 2 Page 163