Who is the "Pieta" group that originally performed this piece with Vivaldi?

The girls from Ospedale della Pietà in Venice

Explanation:

The "Pieta group" that performed with Vivaldi were the girls who lived at the Ospedale della Pietà in Venice. The Ospedale della Pietà was a foundation whose mission was to care for abandoned and orphaned children. Girls who stayed at this place participated in an all-female orchestra that was led by female conductors and featured female soloists. Vivaldi collaborated with the institution for most of his adult life.