Hi,
Six candidates are sitting alongside each other at a long rectangle table facing the same direction.
Six candidates are sitting at a long rectangle table alongside each other facing the same direction.
Six candidates are sitting at the same side of a long rectangle table facing the audience.
'Beside' is more natural here than 'alongside'.
Six candidates are sitting beside each other at a long rectangle table, all facing in the same direction.
Six candidates are sitting at a long rectangle table beside each other, all facing in the same direction.
Six candidates are sitting at the same side of a long rectangle table, and facing the audience. (it's the candidates who are facing the audience, and not the table.
Clive