What is the ruling on congregational prayer (salat al-jama'ah) performed by worshippers between pillars, or outside the mosque in a room where the imam cannot be seen, or in a place opposite the mosque's door with a barrier present?

There is no harm in praying between columns and pillars if the mosque becomes crowded with worshipers. As for praying in a separate room outside the mosque while hearing the Imam's voice without seeing him or the worshipers, or in a place outside the mosque while seeing the Imam or the worshipers, the differing views of scholars on this matter have been previously elaborated upon.