The Gospel doesn’t say that Mary played a part in the Transfiguration, but the Church’s tradition, especially Saint Gregory Palamas in the East, speaks of Mary's role in similar experiences of contact with divine glory, of divinization.
St Gregory Palamas insisted that our divinization is possible and real, and that while remaining creatures, we will participate in divine nature and divine life, through divine energies. Mary was the first to achieve this through grace.
Then, considering the mystery of the Church as a communion of deification, St Gregory Palamas explained that Mary transmits to us this contact with divine life, that transfigures us—the Eastern writers use the term divinize—and makes us capable to participate in divine life.
Source: Marie de Nazareth editorial team