What is the ruling on one who prays behind an imam who is performing a solitary congregational prayer aloud, and what should the imam do in this situation?

If a person prays alone, and then another person joins him to follow him, the Imam remains in his state regarding recitation, audibility (jahr), and inaudibility (isrār). It is recommended for him to intend to lead the prayer (imamah) in order to gain the virtue of congregational prayer, even though his prayer and the prayer of the followers are valid even if he does not intend it.