Does congregational prayer count for someone who began their prayer alone and then others joined them in congregation?

The prayer is valid, and it is permissible to transition from praying alone to leading the prayer (imamate) in both obligatory (fard) and supererogatory (nafl) prayers. This is the view of the Shafi'is and Ibn Qudamah, and it was also stated by Sheikh Ibn Uthaymeen. The Imam gains the virtue of congregational prayer from the moment he merely intends to lead the prayer.