Purity: Standing Position Relative to the Corpse
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 589:
The person performing the prayer must stand with the corpse in front of him. However, if the prayer is being performed in congregation, there is no problem if some of the people do not stand in front of the corpse.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)