The figures at the foot of the Cross are the Virgin and Saint John. Those in the middle distance are probably the Three Maries (Mary Magdalene, Mary the wife of Cleophas and Mary the mother of James).
Antonello's larger version of this subject painted in the same year is closer to the Netherlandish models, with a more narrative character and more distracting detail.
The version here is contemplative in character, with the Virgin and Saint John shown in meditation. The landscape contributes to this mood of meditation.